Thomas Sullivan, also known as Sully, was born in Kilkenny, Ireland. He grew up as a street urchin, learning to fight for scraps of food. Once grown, he became the Kilkenny Bare-knuckles champ, taking on all-comers, and was a very good fighter. In 1788, while working as a prizefighter in Dublin, Tommy refused to take a dive in a fight against the Dublin champion as instructed by the fight's promoter. The promoter had him murdered. Sullivan revived as an immortal and was found by Immortal Rory Colhoun, who becomes his teacher. After training with Rory for two years, and deeming himself ready, Tommy took the time to murder the fight promoter before taking a ship to America, the land of opportunity.
In 1891, Tommy was in San Francisco where he met Duncan MacLeod, who he convinced to participate in a bare-knuckles match for prize money. Mac won the match, but when the place was raided, the promoter claimed he didn't have to pay Tommy the $1000 prize money. Tommy killed the promoter and split the money with Mac, who was unaware of what his friend had done to obtain their winnings.
In the mid 1980s, Tommy was living in St. Louis, Missouri where he was the owner of a boxing gym called Clancy's. In 1994, Tommy Sullivan went on the road to Seacouver to show off his latest boxer, George Belcher, and to get his old friend Duncan to finance the fighter in his bid for a championship.George trained at Charlie's dojo, and Tommy became enamored of Iris Lange, a waitress at a local sports bar. Sully took lessons from Charlie in how to woo her. When he was set up and saw George and Iris in an embrace, he slugged his boxer. Learning that a promoter named Frank Coleman was using Iris to separate George from Sully through blackmail, because of her brothers debts, he killed Coleman. When George also turned up dead because he wanted a new trainer, MacLeod felt he must confront his friend as he can no longer turn a blind eye to Sully's idea of justice.
The two men fought in the boxing ring. Duncan eventually won and took Tommy's head and Quickening. Duncan was visibly upset that he killed Tommy, but believed that he did the right thing.
Tommy believed strongly in the fairness of the game and often stated that he was not a bad person and never killed anyone who didn't deserve it. He saw the world in terms of black and white, and not in shades of gray. One is either innocent or guilty of a crime, there is no in-between.
Tommy used a Spanish Cavalry Sabre and true to his boxing history; he frequently punched MacLeod during their duel.