Garrison was born in Dallas, Texas, on May 23, 1980, and raised in Richardson, Texas. Due to the troubled relationship with his mother, Garrison moved out at seventeen and turned to the family minister, Joe Simpson, who is also the manager and father of pop stars Jessica and Ashlee Simpson. Garrison lived with the Simpson family for a year. He graduated from J.J. Pearce High School in 1998, and at eighteen, he drove to Los Angeles with ambitions to become an actor. During his first day in L.A., he ran into a commercial agent, who sent him to an audition for Eastpak Backpacks. He got the job, which paid him $3,500, money he needed to live and study with several acting teachers, some of which were Joseph Reitman, Belita Moreno and Warner Laughlin. In 2005, he had a major break when he assumed the role of David "Tweener" Apolskis on the television series Prison Break.