The retired racer entered the plea a day before he was set to go to trial on aggravated DWI, drag racing and reckless driving charges. He was sentenced to 90 days in jail, but a judge deferred that in favor of 364 days of supervised probation.
Unser was arrested in September after he was accused of racing another car in his Suburban and driving over 100 mph near Albuquerque. Authorities say Unser smelled of alcohol, his speech was slurred and his blood-alcohol content was twice the legal limit of 0.08.
He was charged with reckless driving and aggravated driving while intoxicated, and later released on released from jail on his own recognizance.
His attorney, Sam Bregman, told the court that his client completed 30 days of inpatient treatment for alcohol abuse.
"Al Unser Jr. is truly sorry for the poor mistake he made last fall," Bregman told The Associated Press. "He's taking steps to make sure it doesn't happen again."
The arrest wasn't Unser's first encounter with the law. In 2007, he pleaded no contest to a driving under the influence charge stemming from a January 2007 crash on a Nevada freeway.
Nevada authorities said Unser's blood-alcohol content was three times the legal limit and he failed field sobriety tests. As part of the plea deal, Unser lost his driver's license for 90 days, was fined $1,000 and had to attend a drunken driving school, a victim-impact panel and undergo alcohol abuse evaluation.
His lawyer at the time said Unser had stopped drinking and was attending Alcoholics Anonymous meetings. Unser also performed more than 100 hours of community service on alcohol treatment-related causes.
In 2002, Unser also underwent treatment for alcohol abuse after his girlfriend said he hit her in the face while he was drunk in Indianapolis. Prosecutors didn't file charges.