Dr Joan Watson is a former surgeon who works as a consulting detective with Sherlock Holmes, having initially been his sober companion. Watson lives (generally) with Holmes in a brownstone house in Brooklyn. ("The Grand Experiment")
Watson has parents and a brother, Oren.
Her mother initially does not approve of Watson's career change from surgeon to sober companion. However, she begins to appreciate it during a dinner with Holmes, where he describes her daughter's work in a positive light. She also has a half sister Lin Wen who is discovered to be her family during "All In"
Watson started out as a surgeon. During one procedure, she made a mistake which in turn cost the patient his life. Subsequently, she resigned from medicine.
After leaving medicine, Watson became a sober companion. Eventually she met a client in Sherlock Holmes, a police consultant from London. During her time with Sherlock, Watson garnered a fascination with his detective work and even on occasion started to think with deductive reasoning.
After her time with Sherlock as her client came to an end, she took him up on his offer to stay with him as his "companion." He regularly trains her, or has others do so, in various skills he uses, including investigation, single stick, lock picking, car-jacking, and lie detection.
She now uses these skills as his assistant consulting detective.
Time with Sherlock Holmes
Joan first meets Sherlock Holmes in "Pilot" where she has been employed by Sherlock's father (sight unseen) to look after him, as Sherlock has just been released from drug rehab. Her role is both to monitor him, and to assist in his rehabilitation. Joan intended to pick him up from the center but Sherlock manages to escape the day of his release.
Sherlock guesses that she used to be a surgeon due to her hands, to which she replies that she isn't one any longer. We later find out that Watson's accidental mistake while practicing medicine resulted in the death of a patient, which she never truly forgave herself for, and after much thought, allowed her license of surgery to expire.
Although Sherlock's father informed her that her services were no longer needed, Joan continued to work with Sherlock unpaid, without telling him that she did not actually have a job anymore. After sometime when Watson admits the truth Sherlock reveals that he knew all along and offers her an opportunity to work alongside him as a detective. In "Possibility Two", Sherlock sends Watson to investigate a dry cleaners, beginning her new role as Sherlock's apprentice and student in deduction.
Joan also sees a therapist who disapproves of her association with Sherlock, believing it has become an obsession and a danger to her safety. Joan reveals her fascination and awe of Sherlock and his abilities to her therapist that she otherwise keeps to herself. Although her therapist recommended she end her association with Sherlock, Joan continued to work for him insisting that he still needed her help.
Joan went to medical school and graduated as the class valedictorian. After that, she became a surgeon for an unknown amount of time until she made a mistake during one operation which ultimately cost the patient his life.
Although she is now retired, Joan's experience in medicine has been useful in a number of Sherlock's cases. In "Lesser Evils", when Epinephrine was used to kill a patient at a hospital, Joan was able to deduce how much time the killer had to flee the scene. Later, she met a young girl who was a patient of an old friend of hers. Joan thought that the girl had Endocarditis, which even her friend did not think was possible, and was proven to be right.
In "The Leviathan", when the blood evidence found at a murder scene came back to a woman who not only had no connection to the victim, but was not even in the country, Joan saw a ribbon in the woman's house, which she recognised as a reward given to bone-marrow donors, which led them to the real killer.
Although she hasn't been a surgeon for some time now, Joan has been shown to still be very skilled in the matter. This was shown in "A Landmark Story", she and Sherlock were secretly investigating the death of a man that had a pacemaker, which was believed to be caused by a heart attack. They broke into the morgue that the man's corpse was held in. After seeing evidence that the man may have been electrocuted, Sherlock declared they had to perform an autopsy and tried handing the surgical tool to Joan, who refused to operate, until Sherlock began making mistakes before even starting. After that, she, albeit reluctantly and grudgingly, opened up the corpse torso without trouble.
During her time with Sherlock, Watson started learning deductive reasoning, which was shown in "The Rat Race" when she thought her date lied by saying he was never married and was right. Her increasing deductive skills were also shown in "You Do It To Yourself", when she saw discolouration in a photo, which even Sherlock, who was sick with a flu at the time, did not spot.
She has displayed such talents that Jamie Moriarty, who originally dismissed her as a pet or mascot, went so far as to engineer the murder of Elena March, an incarcerated crimelord who sought to eliminate Joan.