According to The Sun, McQuillan worked as a doorman around Edinburgh when he was a student.
"James is a big bloke so instead of scrubbing dishes or pulling pints he went in for some bouncing," said a source.
"He was a regular face on club doors, working everything from snooker halls to strip joints and lap dancing clubs. He was a bit worried about his past when he went on the show but he came clean just in case."
The insider added: "I don't think the producers have told Sir Alan because he would get some cheeky comments in the boardroom."
McQuillan, 32, was saved from being fired last week by Sir Alan Sugar's aide Margaret Mountford, who claimed that she saw potential in the former chess champion.