The 26-year-old actor faces up to six years and eight months in prison when he is sentenced in August. He remains free on $100,000 bail.
Garrison was driving a 2001 Land Rover on Dec. 2 when he lost control and rammed a tree. The crash killed Vahagn Setian, 17, a Beverly Hills High School student. Two other passengers, both 15-year-old girls, survived.
Garrison had a blood-alcohol content of .20 percent, more than twice the legal limit for driving, and was under the influence of cocaine, according to police.
He pleaded guilty in Los Angeles County Superior Court to one count of vehicular manslaughter without gross negligence, one count of driving under the influence with a blood-alcohol level of .15 or higher and a misdemeanor of providing alcohol to a minor.
Judge Elden Fox earlier ordered Garrison not to drive a vehicle, drink alcohol, go to bars or contact witnesses or victims of the fatal crash.
The actor's attorney, Harland Braun, said earlier that Garrison was "going to accept responsibility for his conduct. The only question is what is that level of responsibility?"
Garrison's character on Fox's "Prison Break" — Tweener — was killed in the Oct. 2 episode. The Dallas native's other credits include the 2006 film "Crazy" and an episode of the short-lived 2005 ABC series "Night Stalker."