Debbie Madden, 61, of San Mateo, was a civilian criminalist at the crime lab, which was ordered closed last year after she allegedly admitted to taking small quantities of cocaine from evidence there in 2009.
Prosecutors had to dismiss hundreds of drug cases in the city as a result of the allegations.
San Francisco police served a search warrant on Madden's home on Shoreline Drive in San Mateo in March 2010 and found a small amount of cocaine.
She was charged with one felony count of possession of cocaine, and pleaded not guilty when she was arraigned in April 2010.
After that, the case was continually delayed for more than a year by Madden's defense attorney, Paul De Meester, until she finally decided to plead guilty to the charge Thursday at the county courthouse in South San Francisco, Wagstaffe said.
The court date Thursday had originally been scheduled to set the preliminary hearing date, the 12th time that item had been on the calendar over a 13-month period, he said.
De Meester said the case was delayed because he was waiting for San Francisco officials to unseal the search warrant after their investigation into the case was complete. The warrant was unsealed in December, he said.
Following Madden's guilty plea, she was placed on 18 months of deferred entry of judgment and was ordered to complete a drug rehabilitation program, as well as pay $300 in court fees and fines.
"If she attends all the classes and keeps her nose clean, the case gets set aside," Wagstaffe said.
De Meester said the referral to the rehab program "was a good conclusion to all these matters."