Colin Gregg, 75, denies nine indecent assault charges involving four young boys over three decades.
Leeds Crown Court heard from one alleged victim, now in his 60s, who said he was assaulted in 1967 while Mr Gregg was working as a teacher.
Mr Gregg has dismissed the allegations as "evil lies" motivated by money.
The court heard the then teenager was asked by Mr Gregg to help build an adventure playground in Northumberland.
While there, Mr Gregg, from Newcastle, is said to have invited him to drive his car.
In evidence, the witness said he recalled Mr Gregg's was a Saab and accepted the invitation to drive it on a private road.
He told the court he was sitting "squeezed" in the driver's seat in front of Mr Gregg, who then went on to sexually assault him.
The man, who cannot be named, said: "I was shocked. I had to use my elbow with force him to force his hand away.
"It went on for two or three minutes and then I just jumped out of the car and ran off.
"I didn't speak to anyone about it for many years.
"It was so obvious so quickly what he was doing, it was out of the blue. I just wanted to get away. I felt awful."
The charges relate to alleged assaults on four boys aged between 10 and 15 during a period when Mr Gregg was a teacher and head teacher, beginning in 1963.
Earlier, the prosecution said Mr Gregg concealed his true nature behind a "veneer of respectability" and despite a life of distinction, which included being the director of a children's charity, was "an abuser of children".
The trial continues.
The son of the founder of the Greggs bakery chain abused a boy after letting him drive his car, a court has heard.