Freddie Starr has lost his High Court claim against Karin Ward, who said he groped her when she was 15.
Ms Ward, 56, alleges that the assault took place in 1974 behind the scenes of Jimmy Savile's Clunk Click TV show.
Mr Starr, 72, of Studley, Warwickshire, denied the claims and sought damages for alleged slander and libel.
Judge Mr Justice Nicol said the case failed because Ms Ward's testimony was found to be true, and because too much time had lapsed.
The entertainer, who was not in court for the ruling, is now reportedly facing a £1m costs bill and says he has lost £300,000 from shows being cancelled over the allegations.
He sued over interviews given to the BBC and ITV in October 2012, statements on a website and those made in an eBook about Ms Ward's life.
Mother-of-seven Ms Ward - who was a pupil at Duncroft Approved School, in Staines, Surrey, in 1974 - told the court Mr Starr had also made a lewd comment about her chest.
"I carried that phrase with me all my life and it certainly helped to wreck three marriages," she said.
Ms Ward, from Oswestry, Shropshire, also said she was sexually abused by Savile more than once in return for being in the Clunk Click audience at BBC Television Centre, London.
She said while she had been given lithium at her school, which affected her memory, she "very vividly" remembered that Mr Starr smelled of alcohol and cologne, which reminded her of her abusive stepfather.
"It was known back then as a 'goose', when a man would put his hand under a girl's buttocks and give it a squeeze and usually say 'goose' and, at the same time, reach for her breasts and say 'honk, honk'," she told the court.
"He got as far as the 'goose' and I recoiled because, while I expected that kind of behaviour from all men and was used to it, I was distressed because the smell reminded me of my stepfather."
Mr Starr said he did not at first remember appearing on Clunk Click in March 1974, until footage showed him in the studio with teenage Ms Ward behind him.
During the hearing, Mr Starr had rejected the allegation that he had groped the teenager in Savile's dressing-room and denied having "wandering hands".
In evidence Mr Starr also said he did not drink, and never had done.
Dismissing the claim, Mr Nicol, who held the case without a jury, said: "She [Ward] has proved that it was true that he groped her - an under-age schoolgirl."
He added that Starr had "humiliated" her and said that in truth, the entertainer had no recollection of what happened that evening.
Speaking after the ruling, Ms Ward, who has waived her right to anonymity, said she was "relieved" at the outcome.
"For anyone who hasn't yet dared to come forward, I say - do not take this case as a reason not to do so," she said. "Stand tall, it wasn't your fault, and you have a right to be heard without fear or threat of not being believed."
Ms Ward's solicitor, Helen Morris, said after the ruling that Ms Ward had been abandoned by the two broadcasters who interviewed her and had been treated "disgracefully".
She also said journalists from the Newsnight and Exposure television programmes had given evidence, in which they said Ms Ward had performed a public service by being the first victim of Savile to speak out, which resulted in 500 further victims coming forward.
A BBC spokesman said Mr Starr sued Ms Ward over ITV reports and other online material that had been made before the BBC Panorama broadcast in October 2012. He added that the BBC had offered to help Ms Ward with her legal costs and was willing to discuss a "fair contribution" to them.
ITV has declined to comment on the case.
A Crown Prosecution Service investigation into claims brought by Ms Ward and 13 others decided that no charges would be brought.