Garrison, 28, was paroled and picked up at 3:50 am Wednesday from the California Correctional Institution in Tehachapi, California, Lt. Jon Bartelmie said.
He participated in a substance abuse program while at the prison and is eligible for a similar program now that he's been released, Bartelmie said.
Garrison was charged after a December 2006 crash in which he rammed his Land Rover into a tree, killing Vahagn Setian, 17, and injuring two other teenage girls.
He later pleaded guilty to vehicular manslaughter and misdemeanor counts of drunken driving and giving alcohol to a minor.
Garrison was sentenced in October 2007 to serve three years and four months in prison. He was also ordered to serve four years' parole and to pay about $US300,000 ($NZ538,020) in restitution to the victims and their families.
Bartelmie said Garrison was released before serving his full sentence because he received credits for good behavior, which is a common practice.
The actor met the girls at a grocery store and accompanied them to a party. At the time of the crash, he had a blood-alcohol content of 0.20 percent, more than twice the legal limit for driving, and was under the influence of cocaine, according to police.
A civil lawsuit filed by Setian's family and Michelle Ohana, one of the girls injured in the crash, was recently dismissed at the request of the parties.
