Little is known of Tomas' life before the dead walked the Earth, but he was a violent criminal and was sent to the West Georgia Correctional Facility to pay for his crimes. It is unknown exactly what he did to get arrested.
During the initial outbreak, a riot broke out within the prison and Tomas, along with Oscar, Andrew, Axel and Big Tiny, was led into a cafeteria by a guard, given a gun and locked inside. The guard went for backup but never came back, and so Tomas and the others were trapped in the cafeteria.
Ten months later, Rick Grimes and his group of survivors entered the prison and entered the cafeteria whilst fleeing from a swarm of walkers. Confused about what had just happened, Tomas and the other prisoners ask who Rick and his group are, thinking that they are bandits that have raided the prison. Rick tells Tomas and the others about what has happened to the world, which the prisoners were totally unaware of. Realizing that this prison is the safest place to live, Tomas threatens Rick's group, claiming that the prison belongs to the prisoners, and Rick and his group must leave. After an intense discussion, Rick makes an agreement to attain a share of the cafeteria's food in exchange for helping the prisoners clear out walkers from a cell block for the prisoners to live in. During the process of killing walkers, Tomas acts increasingly belligerent towards Rick and the others. Big Tiny gets deeply scratched by a walker and says he feels fine and nothing was going to happen. As the other prisoners ask Rick if there is any way of saving Big Tiny's life, Tomas kills Tiny with no hesitation by brutally smashing his head in. This raises concern for Rick and Daryl, who prepare to take him out if he makes any further hostile moves. Later, the group enters the laundry room with the entrance to the other cellblock, and Rick instructs Tomas to open a single door. After opening both and allowing in the full group of walkers, Tomas justifies his actions by saying "Shit happens."
While the group members are killing walkers in the laundry room, Tomas then intentionally swings his weapon too close to Rick whilst attacking a walker, and then pushes another walker onto him; Rick manages to survive after Daryl stabs this walker in the back of the head. The prisoners finish killing the remaining walkers. Tomas shrugs it off with a comment, to which Rick nods his head and agrees, saying "I get it. 'Shit happens'"; then after they stare each other down, Rick drives his machete into Tomas' skull, killing him.