Darren Hudson, 50, from Morecambe, denied the allegations but was convicted by jurors at Preston Crown Court of 10 counts of sexual assaults after the girl bravely confided in people about her ordeal.
The attacks date back more than five years and took place in Morecambe.
The girl, who cannot be identified for legal reasons, had refused at first when the woman said they should ring the police, because she did not want the defendant to get into trouble.
In her evidence, the woman told jurors she had then confronted Hudson herself, meeting him outside a pub in Morecambe.
The court heard he got into the passenger seat of her car and when she alleged he had been "acting untowards" with the girl, he replied: "Wow, that's mindblowing isn't it?"
Giving evidence for his defence, Hudson denied the offences, or meeting the woman at the pub.
He will be sentenced on October 4.