- Mycroft: "For goodness' sake! I occupy a minor position in the British Government."
- Sherlock: "He is the British Government – when he's not too busy being the British Secret Service or the CIA on a freelance basis. Good evening, Mycroft. Try not to start a war before I get home, you know what it does for the traffic."
- ―A Study in Pink
Mycroft Holmes is Sherlock Holmes' brother and is traditionally older by seven years. It is mentioned in "The Final Problem" that, along with tradition, he remains seven years Sherlock's senior in the BBC adaption. 
Mycroft was born to Mr and Mrs Holmes, the eldest of their three children, with his younger brother, Sherlock, being born seven years later, and their younger sister, Eurus, a year after Sherlock. Eurus was institutionalized at age five after her psychosis caused her to kidnap Sherlock's best friend Victor Trevor in jealousy (she killed him) and also to burn down their home of Musgrave. Their uncle Rudy oversaw this, and Mycroft would later assume the responsibility when he ascended to his position in the British government. He deceived their parents out of kindness to believe she was killed in another fire to spare them to the pain of letting them know what she had become, but would consult with Eurus for her intellectual insight occasionally and still showed brotherly care of her in giving her "treats" in exchange. ("The Final Problem")
In the pilot episode of the series, Mycroft has Dr John Watson brought to him for questioning when he begins investigating crimes with Sherlock and offers him money to help ease his way for information on his brother's activities out of concern for his well-being, which John refuses believing Mycroft a possible criminal. During the meeting, Mycroft, neglecting to mention he's Sherlock's older brother, told John that Sherlock may even consider him his arch-enemy. Sherlock neither confirms nor denies this statement, and introduces them properly at the end of the episode, at which point Mycroft ordered the surveillance of Sherlock be increased to observe how Sherlock and John's partnership will progress and keep them safe. ("A Study in Pink") Mycroft came to Sherlock to ask him to investigate the death of Andrew West and the disappearance of the Bruce-Partington missile project plans, supplying John with further information on West when Sherlock sent him to Mycroft's office. Throughout the episode, Mycroft eventually grew impatient with them, having texted them continuously, but Sherlock eventually solved the murder of West and recovered the memory stick with the plans that West was responsible for, which was afterwards ruined by consulting criminal Jim Moriarty at the end of the episode. Mycroft, however, had stated previously that it was not the only copy of the plans. ("The Great Game")
After Irene Adler, a dominatrix with a passion for making trouble, took a number of compromising pictures of herself with a female member of the royal family, Mycroft had Sherlock and John brought to Buckingham Palace to be briefed on the case and to assign them to recover the photos, which they failed to do. In their effort, American agents tried to get the phone, causing Sherlock to suspect Adler had more than compromising photos and that something big was coming, but Mycroft ordered him to leave the case be from then on and stay out of the way. For some time, he and the British government had been working on a counter-terrorism project with the US government to foil the efforts of terrorists to blow up an aeroplane, and Mycroft didn't want Sherlock in the way after encountering the American agents involved. Mycroft was later informed on Christmas that he might find Adler dead and had "her" brought to the morgue of St Bartholomew's Hospital for Sherlock to identify, after which he gave Sherlock a Christmas cigarette as he seemed sad at the idea of her death. When Sherlock left, Mycroft phoned John to see if there was any danger of Sherlock relapsing on drugs and ordered him to stay with him in case its a "danger night." However, Adler was still alive, and Mycroft didn't know at first, or that Adler was working for Jim Moriarty. On his behalf, she later came to Baker Street and tricked Sherlock into deciphering a MOD official's email that proved the British and US governments knew the plane would be bombed. She texted Moriarty the information, who then informed the terrorists of governments' knowledge of the bombing plot, and Mycroft that their project had been exposed. With the counter-terrorism project foiled, Mycroft brought Sherlock to the plane to confront his unwitting aide in its failure and apologized for pushing him into Adler's path. She then arrived to confront Mycroft and tried to blackmail him and the government with her camera phone's contents, however, Sherlock finally cracked its password, gave it to Mycroft to make up for having ruined the project, and they refused Adler's plea for help despite knowing she won't be able to avoid her enemies for long without the phone's protection. When she was later apparently killed, Mycroft brought her file to Baker Street and gave it to John, them agreeing to tell Sherlock that Adler got herself into a witness protection program in America out of consideration for his brother's unspoken feelings for her. ("A Scandal in Belgravia")
When one of Sherlock's cases led to him breaking into the top-secret military base "Baskerville" using his priority "Ultra" government security pass, an annoyed Mycroft texted Sherlock to ask what he was doing and sent DI Greg Lestrade, Sherlock's Scotland Yard "handler," to keep an eye on him. When Sherlock phoned him later and requested his help, Mycroft pulled strings to allow him 24 hours of unlimited access to Baskerville to help him solve his case. Mycroft is also revealed to have kidnapped Jim Moriarty, and let him go at this end of this episode. ("The Hounds of Baskerville") After Moriarty resurfaced, was found not guilty in the "trial of the century"(which he set himself up for) after extorting the jury, and left Sherlock with a threat of owing him a "fall," Mycroft had John brought to him at the Diogenes Club he frequents to inform him that four top international assassins, who he believed were sent by Moriarty, had moved into flats close to 221B Baker Street, and asked John to watched after Sherlock before he left since Sherlock's sibling rivalry toward him caused him to refuse Mycroft's help. When Moriarty, acting under a false name, later published a kiss-and-tell in a newspaper which claimed Sherlock a fraud, but had his whole life's story within, John came to Mycroft and confronted him about having told Moriarty all this, believing it to have been a betrayal of Sherlock since people would believe the lie given the rest of the story is true. Mycroft, however, had been for some time working on a plan to bring Moriarty down with Sherlock, which included giving Moriarty this information and which they had agreed John would have to be kept out of for reasons of discretion, ("The Empty Hearse") and he here explained around that point, saying Moriarty's key-code was why he had abducted him previously and that he had offered Sherlock's life story to try and get information on it. Their plan ultimately worked and Moriarty was finally defeated(committed suicide thinking he could win when he walked into a trap blindly), in the process of which Sherlock faked his death to go undercover and dismantle Moriarty's international criminal organization. ("The Reichenbach Fall")
After two years of undercover work, Sherlock finally succeeded in dismantling Moriarty's organization but was captured in Serbia shortly afterwards. Mycroft went undercover and rescued him, having learned Serbian in a couple of hours with his extraordinary intellect, and brought him back to London, informing him of a massive terrorist attack the government learned of that he wanted Sherlock to avert. He visited Baker Street for an update, where Mycroft and Sherlock played various games despite the terror alert, which Mycroft found senseless, and had a deduction duel, which Mycroft won before instructing Sherlock to get back to work and leaving. Sherlock ultimately succeeded in thwarting the terrorist plot(an attempt to blow Westminster Palace and wipe out the parliament, headed by Lord Moran who was arrested quickly afterwards), and their parents came into town for a concert Mycroft had promised to take them to, yet he comically called Sherlock to ask him to come and take over at the interval, not enjoying himself at all. ("The Empty Hearse") The day of John and his fiance, Mary's, wedding, Sherlock telephoned Mycroft, who had just finished exercising, and asked him to come down to it, jokingly remarking that there should be a "spectre at the feast", but Mycroft refused, teased Sherlock a little, and asked him to give John and Mary his best wishes before they hung up. ("The Sign of Three") A month after the Watsons' wedding, John phoned Mycroft after discovering Sherlock in a drug den and on drugs again, and Mycroft contacted Philip Anderson and other fans of Sherlock's to search his flat for drugs, and Sherlock explained his drug relapse was part of a plan for a case involving blackmailing media mogul Charles Augustus Magnussen, after which Mycroft sent Anderson and the others away with a threat not to reveal what they have heard and warned Sherlock not to go against Magnussen, or else he will go against him too (because Magnussen was a government asset who is useful, it would later be revealed).
Sherlock refused to listen and assaulted Mycroft when he angered him, after which he departed. When Sherlock was later shot during the course of his case and mysteriously vanished from his hospital room before he could be questioned as to who did it, Mycroft gave DI Lestrade knowledge of two additional bolt-holes where Sherlock is known to go when in need of keeping out of sight. Six months later, on Christmas day, Sherlock had fully recovered from his gunshot wound, and he, Mycroft, John, Mary, and a man named Wiggins, who Sherlock invited, gathered at their parent's house to celebrate (Mycroft reluctantly), where Mycroft and Sherlock again smoke a Christmas cigarette. As they do, Mycroft expressed his relief that Sherlock had apparently given up on the Magnussen case, explaining why he had been opposed to it by calling Magnussen a "necessary evil," and not a "dragon for Sherlock to slay" (Mycroft knows Sherlock thinks of himself as a dragon-slayer). He also urged Sherlock to turn down an offer from MI6 to go on an undercover mission that would prove fatal within six months, which Sherlock complied with before Mycroft confessed that losing Sherlock would break his heart. However, Sherlock hadn't given up on the idea of taking down Magnussen and drugged Mycroft and everyone else present besides John and Wiggins (who was left behind to monitor their recovery). Sherlock stole Mycroft's laptop of state secrets and left with John to Appledore, Magnussen's country estate, to trap him with it, with Mycroft summoning a helicopter and policemen to track it through its GPS locator upon waking up. When they arrived, to his horror, Sherlock murdered Magnussen (because his trap backfired with the lack of blackmail evidence at Appledore), and Mycroft ordered the police present not to kill him. He later convinced other government officials not to incarcerate Sherlock since there is no prison he can be contained safely in, but to instead press him into the MI6 mission previously mentioned as an alternative punishment, which Lady Elizabeth Smallwood authorized. At the airport, Mycroft saw Sherlock off after he was given a last talk with John Watson, however, quickly called him back when every TV screen in the country was hacked to show a video of Jim Moriarty repeating "Did you miss me?" on a loop. ("His Last Vow")
Mycroft and the rest of the highest government officials gathered together with Sherlock in a private meeting, where Mycroft presented doctored security footage of Magnussen's death, which made it look like an over-eager policeman shot Magnussen instead of Sherlock to cover up the murder so he could investigate the Moriarty incident, which Sherlock stated he thought to be a posthumous revenge plan before leaving. Throughout this episode, Mycroft was consulted by Sherlock for information on various people, including whether Moriarty had ever had an interest in Margaret Thatcher (Mycroft said he had been involved in various international crimes, but nothing specific to Thatcher in the last year of his life), and on the elite freelance assassin group AGRA (whose backfired mission in Tbilisi, Georgia Mycroft explained to have been the reason he made the call to end the British government's use of freelancers). Sherlock explained that John's wife, Mary, was once one of its members, which Mycroft didn't know, and asked him to look into the use of the word "ammo" in connection to the Tbilisi incident when one of Mary's fellow AGRA survivors threatened her life. Mycroft agreed but warned Sherlock people like Mary often get "retired" rather than reach old age. When Sherlock found Mary after she ran off to lure her former comrade from her family in an effort to keep them safe and convinced her to return to London after her old comrade was killed by police in Morocco, he phoned Mycroft after having a breakthrough on the word "ammo," which was instead the Latin word "amo," which means "I love," implying perhaps Lady Smallwood whose code name is "Love" and who sent AGRA into Tbilisi may have betrayed them. Mycroft had her security access rescinded and brought her in for questioning on the matter, but she convinced the Holmes brothers she was not involved in betraying either the UK embassy in Tbilisi or AGRA. The person who did, it turned out, was Smallwood's PA, Vivian Norbury, who betrayed them in an effort to cover up her high treason in selling government secrets, which the UK Ambassador in Tbilisi discovered prior to being taken hostage by terrorists in the coup there. Sherlock found her out, and confronted her with Mycroft, the police, and Mary Watson, who was killed by Norbury in saving Sherlock when Norbury tried to shoot him. Mycroft summoned an ambulance, but Mary died quickly before it could arrive. ("The Six Thatchers")
Mycroft was later informed that Sherlock had finally left his flat (after a mourning period), and had him tailed and watched to make sure he was alright. He joined Lady Smallwood in the surveillance room and wondered why Sherlock was wandering about aimlessly, appearing rather cross to Smallwood. When Smallwood remarked that he hadn't made up for falsely suspecting her previously, Mycroft asked how to do so, before later wondering whether Sherlock was with someone. Mycroft phoned John, who eventually answered, let him know Sherlock was out and about and asked to be contacted if Sherlock got in touch. During the brief conversation, Mycroft stated Sherlock was a security concern and that his being Sherlock's brother wouldn't affect that any more than the last time (revealing there was another Holmes sibling). Before he left, Lady Smallwood asked if he was still in touch with "Sherrinford," to which Mycroft replied that he got regular updates and that Sherrinford was secure. Just over three weeks later, Mycroft was contacted by Mrs Hudson, Sherlock's landlady, for an unknown reason, and despite her violation of traffic laws, stood down the police confronting her from over the phone. He later called John again after Sherlock had tried to assault a certain Culverton Smith, and had him brought to Baker Street after going there with some others and finding a meth lab in the kitchen. He stated he was looking for answers as to what drove his brother off the rails, and asked John's impute while wondering why Sherlock had fixated on Smith. John, however, only confronted him about his hinted second sibling, but Mycroft denied there was one (truthfully saying no other "brother") before Mrs Hudson came in and wondered what he and his people were doing.
Answering her and explaining it, Mrs. Hudson laughed at his effort to discover what had been on Sherlock's mind and mocked Mycroft, before insulting him and telling him to leave when John found a prior-recorded DVD from Mary made before she died and sent to Sherlock, to which Mycroft complied. Later, Lady Smallwood gave him her private number in case he might like to get together (no doubt so he can make up for falsely suspecting her before) at the end of this episode. ("The Lying Detective") John later met Mycroft and Sherlock's secret sister, Eurus Holmes, and reported it to Sherlock, who had long since repressed his memories of her from childhood. John advised Sherlock to pressure Mycroft into acknowledging her existence by setting up a frightening, seemingly life and death circumstance in Mycroft's house. He had been trying to enjoying a cinema film at the time, but they got his attention after disabling his security systems and ultimately were able to trick him into acknowledging Eurus, before inviting him to Baker Street. Arriving the next morning, Mycroft was again disrespected by Mrs Hudson, before explaining the traumatic events regarding Eurus from their childhood that led to her incarceration. He explained she was in "Sherrinford," a maximum security facility on an island, before explaining he deceived their parents to believe she was dead to spare them the further pain of the knowledge of what she had become and dismissing the notion that she had left the island or that they had met her. However, a drone carrying a motion-censored grenade and a recording of Eurus' cryptic song from their childhood that she'd sing, which Sherlock started to remember as the conversation progressed, smashed through the window.
Mycroft explained what it was and how long they'd have to clear its blast radius, which they all successfully did, before hijacking a boat and heading to Sherrinford to see whether or not Eurus had ever escaped it. Sherlock got the Security chief's pass and headed for Eurus' cell, while Mycroft and John were questioned by the chief. Revealing himself from his disguise to him, Mycroft then watched over the tapes of psychiatric evaluation sessions of his sister done against Mycroft's orders. Everyone who had spoken to her had been enslaved by her ability to reprogram people's minds, which Mycroft had said Eurus had been able to do since she was five years old. Angry at the Security chief for disobeying his instructions, Mycroft's attention was eventually drawn to the fact that the chief's voice was among those on the tapes and therefore that Eurus had taken over the facility, explaining how she had been able to get out when she wanted. Eurus then had all three of them, along with the chief, locked inside Eurus' cell, and subjected to emotionally traumatic mind games wherein they had to suit her whims by following instructions or solving puzzles in exchange for phone time with a girl supposedly trapped on a plane that would eventually crash, including either Mycroft or John killing the security chief in exchange for Eurus not killing his wife (which she did when they wouldn't and he committed suicide), solving the murder of a man in which three brothers were suspects (Mycroft, initially unwilling to play along with Eurus' games, aided in this one, but after they solved the crime Eurus, who had taken the three brothers captive, killed them all even though only one was guilty), saving Molly Hooper's life by getting her to say "I love you" to Sherlock(her life was never in danger and Eurus simply wanted to make Sherlock emotionally hurt Molly), and lastly trying to force Sherlock to either kill Mycroft or John, which Mycroft tried to make it easy for Sherlock to pick him by insulting Watson insincerely. Despite this, Sherlock refused to kill either, but instead turned the gun on himself and threatened Eurus with a countdown, forcing her to tranquilize him, John, and Mycroft. She didn't hurt him, but Mycroft was afterwards locked in her old cell.
After Eurus stood down eventually and the facility was back under the control of the government, Mycroft spoke to DI Lestrade and let him know he was alright, and he and Sherlock went on to reveal that Eurus was still alive to their parents, who put down Mycroft for deceiving them about her, with Sherlock defending him. He stated she was now catatonic and wouldn't respond to anyone when they asked to see her, yet Sherlock was able to get her to by playing the violin outside her cell, to which she responded by starting to play with him. In the time that followed, Mycroft and their parents came to see them play on one such occasion. ("The Final Problem")
Mycroft claims to merely "occupy a minor position in the government." However, according to Sherlock, he functionally is the British Government, acting as the control centre or "clearing house" for all government actions and decisions. There are signs throughout the series that Sherlock Holmes is right about Mycroft's high position in the UK Government, such as his ability to control CCTV cameras at will. He also is shown to have the ability to hack into cash machines and make them display messages he wants the user to see. Reflecting his high position is his priority level "Ultra" and clearance level higher than the Cabinet Office or even the Prime Minister: him and a group of peers with certain codenames are the only people to know what really happened at Appledore and doctored the footage that will go public and even managed to clear Sherlock of his crimes. He is also implied to have a great say over certain policies: the fact that the British Government stopped using freelance agents for missions after the incident in Tbilisi is his initiative. With his priority level, he has access to MI5's top secret archives.
Mycroft has a habit of kidnapping people to talk to them, instead of using more conventional methods. He has very good connections; it is implied by Sherlock that he knows the Queen, when Sherlock starts to play "God Save the Queen" on his violin, and he is able to pull strings to allow Sherlock access into a top secret military base. In this episode, it is also asserted that his priority level is "Ultra". Unlike his brother, Mycroft is known to only text when he cannot talk.
Mycroft occasionally warns John when he thinks that it is a "danger night" with Sherlock ("A Scandal in Belgravia"), and has John search the house. On those nights, he needs John to stay at home with Sherlock and look out for him. However, Sherlock knows that John has been searching his stuff, and may be aware of what Mycroft has done (and what he has asked John to do).
- "Balance of probability, little brother."
- ―Mycroft to a young Sherlock [src]
Despite the hostility/rivalry the latter maintains toward him, Mycroft cares greatly for his brother, Sherlock, although his ways of watching out for his brother are unorthodox in the extreme. He is an almost entirely socially detached and calculating individual with an exceptional degree of self-control.
After he has first witnessed Sherlock and John working together, he tells his assistant to update Sherlock and John's surveillance status to level 3, implying that he already had governmental surveillance on Sherlock. In tandem with his high intelligence, his self-confidence and mental resilience are almost inhuman, as he seems to be practically immune to even his own brother's probing and he was even able to resist Eurus own ability to instantly reprogram a person despite Eurus being more intelligent. He is, however, not above petty or childish arguments with Sherlock, claiming that the latter's petty feud with him was "childish" and upsetting their mother, and he also argued with Sherlock in front of one of his peers and John in Buckingham Palace to force him to wear pants. Like Sherlock, Mycroft also smokes cigarettes albeit low-tar only, with him carrying a pack at any moment and able to enjoy chatting over smoking, usually without his parents finding out.
Mycroft is shown to be much more capable of faking or maintaining a degree normalcy than his brother, whereas Sherlock is ostracized for his behaviour, Mycroft is highly respected by his peers. Though Mycroft is apparently just as cold as his brother, once informing him categorically that caring was not an advantage, he still strongly reacts to the loss of a human life, as seen in his forced part in Eurus' game at Sherrinford. He refused to take the life of Sherrinford's governor when offered the gun, felt the condemnation game over the lives of the three Garrideb brothers to be insane and inhuman, worried over the imminent death of Molly Hooper's life, and acted to preserve the lives of the people in the city which were threatened by the 'plane' that Eurus had set up.
He is thought to be very lonely and isolated. Sherlock has said this to him too, as even Sherlock has friends but Mycroft has none (that he considers friends at least). Mycroft believes himself to be "living in a world of goldfish", showing how he believes himself to be surrounded by people of much lower intelligence.
However, despite his lack of romantic interests, it is suggested that he is not a virgin unlike his brother Sherlock. When in Buckingham Palace, upon telling his brother not to be alarmed as Irene Adler's job was to do with sex, Sherlock replied "Sex doesn't alarm me," to which Mycroft sarcastically asked him "How would you know?". To make up for his accusation of treason, Lady Smallwood has taken up Mycroft on this regard. On other occasions, Irene Adler had informed the Holmes brothers that Moriarty's nickname for Mycroft was "The Iceman" whereas his nickname for Sherlock was "The Virgin." Also, during the wedding of John and Mary, Sherlock had phoned Mycroft just after Mycroft had finished exercising and asked him why he was out of breath. He then stated that he had either caught Mycroft in a "compromising position" or he had been working out again. He might work out primarily to keep his body healthy and thus his mind sharp as it seems he is quite physically weak. This is demonstrated when he patronizingly referred to Sherlock as "brother mine" and Sherlock effortlessly twisted his arm up his back. John even warned Mycroft that Sherlock could easily injure him severely and Mycroft reluctantly departed, indicating he is capable of picking and choosing his confrontations, a trait Sherlock either lacks or dismisses.
Again, despite his cold and seemingly unsympathetic personality, he cares greatly about his family and those close to him, especially his younger brother Sherlock, as he smiled impulsively upon being surprised by a home video of him and his family as a child, and at seeing a young Sherlock tackle his younger self in a hug. In the first episode he had told John that he "worries about him (Sherlock) constantly." In another episode, Sherlock and Mycroft are seen to be smoking outside their parents' house on Christmas day, where Mycroft says to Sherlock "Your loss would break my heart." Mycroft also cares a lot about John, as in Eurus' last game at Sherrinford of trying to force Sherlock to choose between killing Mycroft or John in order to be granted another chance at saving the 'plane' from crashing into the city, Mycroft, despite the asset his superior intellect would contribute, tries to manipulate Sherlock into killing him over John out of regard for them both, a gesture that Sherlock himself recognized and admitted made it even harder for him to raise the gun. When Sherlock was prepared to kill himself, to wreck Eurus's last game, as well as spare John and Mycroft, Mycroft was shown to be horrified, and stepped forward, as if to try and stop him, showing just how much he cares for his brother. He also cares about his parents very much, as he keeps them informed as to whether Sherlock may have relapsed on drugs and, though definitely misguided, he also lied to them about Eurus and said she died in a fire after being taken away, feeling this was kinder than telling them what she had become (a complete psychopath). This led to their outrage when he finally confessed the truth to them, and Mycroft was very ashamed as they reproached him. However, they forgave him for it, attending a violin recital of Sherlock and Eurus in Sherrinford with Mycroft, wherein his mother placed her hand on his, showing they harbored no animosity for his previous deceit.
In contrast to his relationship with Sherlock, Mycroft's feelings to his youngest sister Eurus were more estranged. Though he recognized the outstanding genius of her superior intellect, Mycroft was aware of Eurus's disturbing personality and actions. Aware of the dangers posed by his sister's psychosis coupled with her abilities, Mycroft supported Eurus's secret institutionalization at Sherrinford with their uncle Rudy's help. Mycroft regards Eurus as a danger first and foremost and conducts himself accordingly, being resistant and unwilling to becoming subject to her manipulations. The mere mention of her, and recalling memories of Eurus, brought even higher guard and dread than even Charles Augustus Magnussen's name. In turn, Eurus had a singular preference for Sherlock and seemingly holds Mycroft with no regard beyond Sherlock's attachment to Mycroft and in comparison with Sherlock's attachment to John. Despite this, Mycroft does hold a measure of sentiment for his younger sister, occasionally speaking to Eurus and exchanging consultations of her genius insight with 'treats' such as a violin and, despite the extreme danger that in part came from it, a genuine unsupervised conversation with Jim Moriarty for five minutes. Though his mother abhorred Mycroft's actions of secretly institutionalizing Eurus for decades, Sherlock recognized that Mycroft had done his best considering everything regarding Eurus. Mycroft also harbors a love for vintage cinema in his home, silently quoting the lines by one of the leads.
- Genius-Level Intellect: Mycroft is immensely intelligent and possess incredible analytical and deduction abilities that surpass even his younger brother Sherlock, with Sherlock saying he has always been the "smart one" between them and, as children, even made Sherlock think he was a fool, and is only rivaled by Charles Augustus Magnussen. He is the second most intelligent human on Earth. However, while Mycroft's intellect was professionally referred to as remarkable, he is still not quite as smart as Eurus, who was deemed to be an era defining genius beyond Newton and could perceive things beyond even Mycroft's own scope. However, Mycroft's genius was still enough that he is one of the only three known persons on the Earth, the other two being Sherlock and Moriarty, who can safely interact with Eurus without succumbing to her manipulation abilities and even Eurus admits Mycroft was very clever, able to pick up things on her scope if she went a bit slow for him.
- Deduction: Mycroft's powers of deduction are extremely high, surpassing even that of Sherlock, who is renowned to be the greatest detective in the world, and is beyond the standards of Charles Augustus Magnussen even though Mycroft relied on Magnussen most heavily. His superiority to Sherlock was shown when he discovered what had happened to a hiker by simply "glancing over the police reports". At one point, he and Sherlock have a deduction battle and, as always, he wins. Mycroft was also able to correct Sherlock's deduction of John and the lilo, by saying it was the sofa, and could perceive that Alex Garrideb was short-sighted but had corrective laser surgery, which Sherlock's deductions had missed, from simply looking at his clothes.
- Mental Resistance: Due to Mycroft's inhumanly intelligent mind, he possesses extremely strong mental resistance and, coupled with his confidence, he is practically immune to even Sherlock's mind probing and even resisted the more intelligent Eurus' ability to manipulate others instantly despite Eurus's superior intellect, a feat that only 2 other men have ever accomplished, Sherlock Holmes and James Moriarty.
- Expert Deceiver: Though not a skill he enjoys or prefers using much, Mycroft proved to be an excellent deceptor, being able to successfully fool his parents of what truly happened to Eurus for years with ease.
- Multilingualism: Mycroft's tremendous intellectual aptitudes and geniuses makes him highly adept at learning new languages, evidenced when he went to save Sherlock after he was captured in Serbia. Mycroft quickly fully mastered Serbian in 2 hours and was able to speak it with enough fluency to fool Sherlock's captors completely with ease long enough to rescue his brother.
- "Bravery is by far the kindest word for stupidity, don't you think?"
- ―Mycroft to John [src]
- "My brother has the brain of a scientist or a philosopher, yet he elects to be a detective. What might we deduce about his heart?"
- ―Mycroft to John [src]
- "Too much history between us, John. Old scores. Resentments."
- ―Mycroft to John [src]
- "Nicked all his Smurfs? Broke his Action Man?"
- ―John to Mycroft [src]
- "If you seem slow to me, Sherlock, can you imagine what real people are like? I'm living in a world of goldfish."
- ―Mycroft to Sherlock [src]
- "I am the smart one."
- ―Mycroft to Sherlock [src]
- "Mycroft is the name you gave me, if you could possibly struggle all the way to the end."
- ―Mycroft to his mother, when addressed by her as "Mike"
- "Both of us thought you were an idiot, Sherlock. We had nothing else to go on, until we met other children."
- ― Mycroft to Sherlock
- He is on a diet, which Sherlock comments about frequently. Mycroft is also made fun of for his working-out. This is due to the fact that Mycroft is traditionally portrayed as overweight. This enables a joke in the first episode where Mycroft is implied to be the lean, menacing Moriarty before the reveal that he is Mycroft, that he has lost weight and that Sherlock has yet to encounter Moriarty. In the Christmas special set in the Victorian world of the books, Mycroft is indeed overweight as he is in the books.
- He wears a ring on the ring finger of his right hand. It is unknown if this is symbolic of anything. Show co-creator and actor of Mycroft, Mark Gatiss, has stated that it is not a wedding ring, but that he likes to think there is a story behind it. It has also been stated that it was because Gatiss did not wish to take off his wedding ring, which is why Mycroft sports it. 
- He does not appear during the "Unaired Pilot", and his lines during the ending scene are given to DI Greg Lestrade.
- Mycroft is a member of the Diogenes Club, a private establishment that requires silence at all times, as in the canon.
- Mycroft has an assistant called Anthea who appeared in the first and third seasons
- Irene Adler revealed that Jim Moriarty's nickname for him was "The Iceman".
- He and Sherlock have a third sibling, as he was speaking to another government official about the country needing Sherlock Holmes. As he dismissed the official about feeling sentiment for Sherlock, he briefly told the official "You know what happened to the other one". In Series 4, their third sibling was revealed to be a sister, Eurus Holmes.
- It is revealed that Mycroft might just be smarter than Sherlock in many respects, his presence in Sherlock's mind palace after the latter was shot corroborates this even further.
- He is considered by those in his rarefied circle to be the most powerful person in England.
- Sherlock tells John that Mycroft has OCD.
- Mycroft is fluent in Serbian. He appears to be very adept at learning languages, claiming to learn Serbian in only a couple hours, with Sherlock revealing he used to be even faster when he was younger.
- In any case of death for Mycroft, he would prefer not to be shot in the head, as he had promised his brain for the Royal Society.
- Moffat, Steven (writer) & McGuigan, Paul (director). (25 July, 2010). "A Study in Pink". Sherlock (2010). Series 1. Episode 1. BBC One. Cite error: Invalid
<ref>tag; name "SP" defined multiple times with different content
- Thompson, Steve (writer) & Haynes, Toby (director). (15 January, 2012). "The Reichenbach Fall". Sherlock (2010). Series 2. Episode 3. BBC One.
- Moffat, Steven (writer) & McGuigan, Paul (director). (1 January, 2012). "A Scandal in Belgravia". Sherlock (2010). Series 2. Episode 1. BBC One.
- Gatiss, Mark (writer) & McGuigan, Paul (director). (8 January, 2012). "The Hounds of Baskerville". Sherlock (2010). Series 2. Episode 2. BBC One.
- Gatiss, Mark (writer) & McGuigan, Paul (director). (8 August, 2010). "The Great Game". Sherlock (2010). Series 1. Episode 3. BBC One.
- Gatiss, Mark (writer) & Lovering, Jeremy (director). (1 January, 2014). "The Empty Hearse". Sherlock (2010). Series 3. Episode 1. BBC One.
- Thompson, Steve; Moffat, Steven; Gatiss, Mark (writers) & McCarthy, Colm (director). (5 January, 2014). "The Sign of Three". Sherlock (2010). Series 3. Episode 2. BBC One.
- Moffat, Steven (writer) & Hurran, Nick (director). (12 January, 2014). "His Last Vow". Sherlock (2010). Series 3. Episode 3. BBC One.
|Sherlock Holmes (Benedict Cumberbatch) • John Watson (Martin Freeman) • Mycroft Holmes (Mark Gatiss) • Jim Moriarty (Andrew Scott) • Greg Lestrade (Rupert Graves) • Molly Hooper (Louise Brealey) • Mrs Hudson (Una Stubbs)|
|Irene Adler • Mary Watson • Charles Augustus Magnussen • Eurus Holmes|
|"A Study in Pink" • "The Blind Banker" • "The Great Game"|
|"A Scandal in Belgravia" • "The Hounds of Baskerville" • "The Reichenbach Fall"|
|"The Empty Hearse" • "The Sign of Three" • "His Last Vow"|
|"The Six Thatchers" • "The Lying Detective" • "The Final Problem"|
|Unaired Pilot • "Many Happy Returns" • "The Abominable Bride"|
|"Sherlock: The Network" • Manga|