Captain Thomas "Tommy" Gregson is the captain of the 11th Precinct in the New York City Police Department, and a primary character in Elementary. He works alongside Detective Marcus Bell. He is Sherlock Holmes' police contact and supervisor.
In "Pilot", he states that after the 9/11 attacks he was sent over to London to observe how the New Scotland Yard operate counter terrorism which is where he met Sherlock who was, at the time, working homicide.
In "The Rat Race", Joan, who has been out of contact with Sherlock for too long, is afraid he relapsed and has no choice but to reveal herself as a sober companion for Sherlock to Gregson. Gregson later reveals to Sherlock that he had known about his problem for some time. He agrees to keep a lid on this information from everyone else.
In "One Way to Get Off", Holmes helped Gregson investigate a new killer who repeated the methods of serial killer Wade Crewes, who had been arrested for the previous crimes in 1999. Although the subsequent investigation revealed that Gregson's ex-partner had planted the evidence that convicted Crewes, Sherlock was also able to discover that the new murders were being commited by Crewes' son, Sean Figueroa, to try and clear his father, allowing Sherlock and Gregson to halt the new crimes and confirm Crewes' guilt.
In "Dead Man's Switch", after witnessing the murder of blackmailer Charles Milverton, Sherlock went to ask Gregson for help in deciding what to do given the complexities of the current case, citing Gregson as an investigator he both respected and admired; if Milverton's death was exposed, his accomplice was under orders to release Milverton's blackmail material on all his 'clients'. Their subsequent investigation discovered that Milverton had been killed by his new partner after eliminating the old one.
In "An Unnatural Arrangement", the team meet Gregson's wife- who he is currently undergoing a trial separation from- when her house is broken into. The subsequent investigation reveals that the break-in to Gregson's house was a mistake, but Gregson and his wife decide to give their marriage another chance.
In "Up to Heaven and Down to Hell", it is revealed that Gregon has officially separated from his wife and is now dating Paige, a former member of the NYPD who runs a bookstore after her unit was involved in a scandal where some of them were taking bribes, Paige leaving to escape the scandal. When they run into Joan while on a date, Paige breaks up with Gregson, officially so that his image won't be tainted by dating her, but Joan and Gregson learn that Paige is actually suffering from MS, Joan providing Gregson with some background information about the disease to help him decide if he wants to stay with Paige despite this.
In "Absconded", Gregson is offered a promotion to Deputy Chief due to the good work of his unit. It is also mentioned in that episode that he served at the 14th Precinct as a newly-promoted Detective and was made head of the Major Case Squad at age 40.
- Gregson's name was initially given, in press information prior to the show airing, as "Captain Tobias "Toby" Gregson". However, his name was later confirmed to be "Captain Thomas "Tommy" Gregson".
- He has daughters, though it is not specified how many. ("Dead Man's Switch") One of his daughters is Hannah, an ambitious patrol officer with the 15th Precinct. ("Rip Off")
- He and his wife Cheryl are not living together though they are not divorced. ("An Unnatural Arrangement") By Season 3, they are officially divorced.
- As seen in "End of Watch", Gregson is a recipient of the U.S. Flag Bar, World Trade Center Bar, NYPD Medal of Honor, NYPD Medal for Valor, and the NYPD 170th Commemorative Breast Bar.