Garrison pleaded guilty to charges of vehicular manslaughter without gross negligence, driving under the influence and providing alcohol to a minor in August.
In December 2006, Vahagn Setian was a passenger in Garrison's Land Rover when the actor lost control of the vehicle and crashed after leaving a pre-Christmas party. The 17-year-old was later pronounced dead in the hospital.
On Wednesday, Garrison was jailed at Beverly Hills court in California for almost four years.
According to Web site TMZ, Garrison issued an apology to Setian's family before he was handed his sentence.
He must also pay close to $300,000 to the Setian family and two other passengers.
Prior to his sentence, Garrison was undergoing an enforced 90-day psychiatric evaluation behind bars at the California Institute for Men in Chino, California.
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