Smith's attorney entered the plea on her behalf Monday without her appearing, said Orange County district attorney's spokeswoman Susan Schroeder.
She was also ordered to perform 80 hours of community service and pay $1,300 in fines and penalties.
Smith, 19, was arrested in March for investigation of felony driving under the influence of alcohol after crashing a Volkswagen Beetle into another car on the San Diego Freeway, according to a California Highway Patrol report.
The driver of the other car suffered minor injuries as did a passenger in the car driven by Smith, the report said.
Although Smith was arrested for investigation of felony DUI, the charge was lessened to a misdemeanor because the injuries were not serious, officials said.