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 (EUR953) 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.
Brenton Tarrant, the shooter from New Zealand's Christchurch, was not a lone wolf. The West has missed out an important point - the formation of organised Christian extremism
Those who convientenly blame Muslims and Islam for "extremism" and "terrorism" should rethink and read the living history for truth, honesty and justice
The Moroccan occupation remains the last case of decolonization in Africa. The Moroccan military invaded in 1975 despite a clear ruling issued by the ICJ.