The Los Angeles County jury delivered the verdict on Tuesday in favour of Judy Huth, who is now 64, and awarded her 500,000 dollars (£407,327).
Ms Huth said the fact that jurors believed her story meant more than the sum of money or the fact that she did not win punitive damages.
“It’s been torture,” she said of the seven-year legal fight. “To be ripped apart, you know, thrown under the bus and backed over. This, to me, is such a big victory.”
Jurors found that Mr Cosby intentionally caused harmful sexual contact with Ms Huth, that he reasonably believed she was under 18, and that his conduct was driven by unnatural or abnormal sexual interest in a minor.
The jurors’ decision is a major legal defeat for the 84-year-old entertainer who was once hailed as America’s dad.
It comes nearly a year after his Pennsylvania criminal conviction for sexual assault was thrown out and he was freed from prison.
Ms Huth’s lawsuit was one of the last remaining legal claims against him after his insurer settled many others against his will.
Mr Cosby did not attend the trial or testify in person, but short clips from a 2015 video deposition were played for jurors, in which he denied any sexual contact with Ms Huth. He continues to deny the allegations through his attorney and publicist.
Jurors had already reached conclusions on nearly every question on their verdict form, including whether Mr Cosby abused Ms Huth and whether she deserved damages, after two days of deliberations on Friday.
But the jury foreperson could not serve further because of a personal commitment, and the panel had to start deliberating from scratch with an alternate juror on Monday.
Mr Cosby’s attorneys agreed that he met Ms Huth and her high school friend on a Southern California film set in April of 1975, then took them to the Playboy Mansion a few days later.
Ms Huth’s friend Donna Samuelson, a key witness, took photos at the mansion of Ms Huth and Mr Cosby, which loomed large at the trial.
Ms Huth testified that in a bedroom adjacent to a game room where the three had been hanging out, Mr Cosby attempted to put his hand down her pants, then exposed himself and forced her to perform a sex act.
She filed her lawsuit in 2014, saying that her son turning 15 — the age she initially remembered being when she went to the mansion — and a wave of other women accusing Mr Cosby of similar acts brought fresh trauma over what she had been through as a teenager.
My client deserves to have Mr Cosby held accountable for what he did
Ms Huth’s attorney Nathan Goldberg told the jury of nine women and three men during closing arguments on Wednesday that “my client deserves to have Mr Cosby held accountable for what he did”.
“Each of you knows in your heart that Mr Cosby sexually assaulted Miss Huth,” Mr Goldberg said.
A majority of jurors apparently agreed, giving Ms Huth a victory in a suit that took eight years and overcame many hurdles just to get to trial.
Mr Cosby’s spokesman Andrew Wyatt said they would appeal the verdict and he claimed the defence won because Ms Huth did not win punitive damages.
Outside court, a group of four women who said they represented dozens of Mr Cosby’s sexual abuse victims celebrated the verdict.
Chelan Lasha, who testified against the actor in the Pennsylvania case, was in tears. She said he had drugged and assaulted her in the Elvis Presley suite of the Las Vegas Hilton when she was 17-years-old.
“He’s a boogey man, he’s a sexual predator, he’s a deviant,” Ms Lasha said.
“He’s horrible. It’s one more victory. I came here to stand with Judy. I’m proud of Judy. I’m glad for the outcome.”