Tommy Gregson started working for the NYPD thirty years ago. He was assigned to Scotland Yard a few months after the September 11 attacks to observe their counter-terrorism bureau. Although Sherlock was mostly assigned to homicides, their paths still crossed a few times. Ten years later, when Sherlock moves to New York City where Gregson currently lives and works as Captain of the New York Police Department, he hires him as a consultant detective. ("Pilot")
He was married to Cheryl Gregson, and has several adult daughters. ("An Unnatural Arrangement") One of them is named Hannah and she works as a police officer at the 12th Precinct and wishes to become a captain like him. ("Rip Off")
When Sherlock is kidnapped, Joan becomes worried when he doesn't answer her messages. Suspecting that he may have relapsed, she tells Gregson that she's a sober companion for recovering drug addicts, and that Sherlock is her client. After Sherlock is found, Gregson tells him that he knew he was in rehabilitation before he hired him to work for the NYPD, and that since his services as a consulting detective hasn't altered, he doesn't see a problem with it. ("Rat Race")
|It is revealed after a break-in in his house that he and his wife are on a trial separation. Cheryl is tired of Gregson's job being prioritized over her, but Gregson thinks the trial separation is just a break for Cheryl and that she'll come back. After realizing why his wife wanted a separation, he lets her know that she deserves better and that he is going to use the separation to figure out how to give it to her. ("An Unnatural Arrangement")
|Gregson physically assaults his daughter’s partner after learning he was abusive with her in the context of a romantic relationship. His daughter, Hannah, is not pleased with way he handled the situation, feeling he has caused talk among her fellow officers. Gregson discusses the situation with Kitty Winter and she helps him understand his daughter’s position, and her desire to keep from appearing as a victim to her colleagues. Gregson eventually agrees to shake the officer's hand in front of the precinct. The officer tells Gregson he is quitting the force and seems scared, Gregson then realizes that Kitty intimated the officer. ("Rip Off")
|Gregson's new girlfriend Paige Cowan, is a former police officer who was forced to resign. When Joan runs into them at a coffee shop she inadvertently triggers their break-up, Paige had wanted to keep their relationship a secret from Tommy's work colleagues. It is later revealed that the larger reason Paige broke things off was because she was recently diagnosed with Multiple Sclerosis. Gregson wants to be with Paige, even with her health issues. Gregson convinces Paige to resume their relationship. ("Up to Heaven and Down to Hell")
|As a favor to Gregson's girlfriend Paige, Sherlock investigates her doctor's staff, and finds evidence of wrong doing. However, in doing so he uncovers that Paige is having financial difficulties. Sherlock suggests that Gregson marry Paige so that she may be covered under his insurance, Gregson admits that he loves Paige and wants to marry her, but she isn't keen to do so. Sherlock arranges a meeting between Gregson and Paige after speaking to her about marriage. Apparently Sherlock managed to lower her defenses and then told Gregson they should get married because Gregson said he wanted it. Sherlock even provided a selection of rings. ("Render, and Then Seize Her")
|When the city wishes to recognize the Major Crimes division, Gregson wants Joan and Sherlock included in the commendation. Sherlock is against this and tries to sabotage Gregson's efforts with his higher ups, while Joan tries to encourage the recommendation. Joan eventually convinces Sherlock to accept the honor. ("Henny Penny the Sky Is Falling")
Every photo of Tommy Gregson on this wiki can be seen here.
Arthur Conan Doyle's Inspector Gregson on Wikipedia, which Elementary's Tommy Gregson is based on.