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. In 2005, he had a major break when he assumed the role of David “Tweener” Apolskis on the television series Prison Break.

In 2007, Garrison pleaded guilty to vehicular manslaughter, drunken driving and providing alcohol to a minor after a crash in December 2006 that killed 17-year-old Vahagn Setian, who had been a passenger in his car. Two 15-year-old girls, also passengers, were also injured in the crash. Garrison met Setian and the girls outside a liquor store then attended a high school party with them as a 26-year-old before jumping a curb and driving into a tree in Beverly Hills. He had cocaine in his system and his blood-alcohol level was 20.5 times the legal limit in California. He was sentenced to 40 months in jail on October 31, 2007. The actor was released from prison in April 2009, and served four years of parole. He was also ordered to pay $300,000 in restitution to the victims and their families.

In 2012, Garrison was accused of slapping his former girlfriend Ashley Mattingly at her apartment building in Los Angeles. Garrison pleaded no contest to the charge and in return avoided having to spend any more time behind bars. In 2013, he was sentenced to attend self-help classes, 52 Alcoholics Anonymous meetings and complete eight hours of community service.

In 2018, Garrison welcomed a baby girl, Linden Rose, with his fiancée Mary Kaitlin.