Judy Huth testified to the civil trial that Cosby, 84, had forced her to perform a sex act on him at the venue when she was 16 years old.
The jury also ruled that Cosby must pay Ms Huth, who is now 64, $500,000 (£407,000) in damages.
Less than a year ago, Cosby walked free from prison after his sexual assault conviction was thrown out.
More than 50 women have come forward with abuse allegations against the man once known as America's Dad, charges he denies.
The civil case against Cosby in Santa Monica, California, was the first to go to trial and represented one of the last remaining legal actions against him. Filed in 2014, it was delayed for years by Cosby's criminal proceedings.
Ms Huth's suit alleged that he brought her and another underage friend to the infamous mansion, lured her to a bedroom and forced himself on her.
When she resisted, he forced her to perform a sex act on him, she testified.
In tears on the stand, Ms Huth told the court she felt "mad, duped and foolish" after the incident, as her lawyers presented a photo of her and Cosby inside the mansion.
Cosby was not required to appear in person at the trial and never did, but the court saw a 2015 deposition by Ms Huth's attorneys, in which the comedian said he did not remember her and would not have pursued sex with a minor.
Lawyers for Cosby sought to poke holes in Ms Huth's claims, noting she had initially said she was 15 at the time of the incident, but changed that detail weeks before the trial.
They claimed Ms Huth had offered her photo with Cosby to tabloids and others in search of a payout.
Jurors were also shown a videotaped 2016 deposition of late Playboy founder Hugh Hefner - a close friend of Cosby in the 1970s - who said it would have been "very unusual" for underage girls to be allowed inside the mansion.
Known to millions for his portrayal of TV dad Cliff Huxtable on The Cosby Show, the comic has faced accusations of sexual assault, misconduct and rape stretching back to the mid-1960s.
In 2018, he was convicted of drugging and molesting a woman at his home in 2004 and sentenced to three to 10 years in prison.
The conviction, however, was overturned by the Pennsylvania Supreme Court last year. He was released from prison after serving nearly three years.