What got Thomas into prison isn't known. He was an inmate that was let out of his cell by the guards and locked in the cafeteria for he protection at the start of the zombie uprising. He stayed in there for four months or so with Dexter, Andrew, and Axel.
Written by Robert Kirkman and drawn by Charlie Adlard.
Major Story Arcs
In the Jail
Even with limited company and tight space, Thomas doesn't come off well with his cafeteria-mates and none of them are sure why. When Rick and Tyreese find the men Thomas says he's in jail for tax fraud - something he claims it wasn't his fault. He lives among the group without really taking any part in the daily activities besides helping the women pack their things into their rooms/cells. He does have a quick visit with Patricia, who tells him that she just broke up with Otis.
The deaths of Susie and Rachel Greene rocks the dynamic within the prison and Dexter is suspected of their murders (having been incarcerated for murder) and locked in a cell. While Dexter is locked up, Thomas finally makes an appearance in the laundry room with Andrea. While refusing to help her with the laundry he pulls out a knife and tells her he plans on cutting off her head instead. Andrea goes for her gun but Thomas wrestles it away. After a struggle in which Thomas cuts Andrea's face and cuts off her left earlobe she is able to escape. She runs outside with Thomas in hot pursuit. Rick immediately jumps into action and tackles Thomas. Realizing that Thomas is the one who killed the Greene girls he begins pummeling Thomas' face. Tyreese pulls Rick off of Thomas before he can kill him.
Thomas is initially held in the cafeteria's freezer. Rick realizes a problem: the inmates had used the freezer as their toilet since it was airtight; Rick is afraid Thomas will suffocate to death, a death too easy for him. (Tyreese just liked that Thomas would have to wallow in his own crap.) The men pull him out and place him in a cell to await a hanging. Oddly enough, Patricia thinks there has been a huge misunderstanding. She feels Thomas is crazy and not evil and decides to help him escape. She gets him up and out of his cell, which is enough time to gather his strength and begin strangling her. She scratches at his face and he falls back with a yell. The commotion prompts Rick's group to assemble. Tyreese puts a gun in Thomas' face. He is shot twice from behind, however, by Maggie. She puts an extra four shots into him for good measure. To dispose of Thomas' body, Hershel requests that Rick has it thrown to the zombies outside the prison's fences so that he can watch the body ripped apart to get a bit of closure.
Thomas' body is seen later when another zombie gets its intestines stuck on Thomas' ribcage.