The revelation came as part of the talk show's weeklong "Big Secret" segment, and Osbourne, 60, was the first of the co-hosts to open up with her surprising confession.
"It's not like it's a dirty little secret," she said nervously before telling her story.
"When I was 25, I just moved to America," she began. "I had met my husband [Ozzy], but we weren't together. We were just friends. I didn't have a boyfriend. I never did date a lot at that time, anyway, because I was not very dateable."
A friend invited her to a club where she saw Leno performing his comedy routine, and Osbourne told her pal that she thought he was "very nice, very funny."
The pair decided to prank call him. "He liked my voice," she admitted, so she kept calling back and pretended to be a wrong number.
"One thing led to another," she continued. "He came to my house and met me and then we had a little fling. The fling was more fling for me and not fling enough for him."
"A couple of months into it he brought around the real love of his life for me to meet. She was lovely and they took me to Fat Burger and they showed me around town," Osbourne said. "They were so kind to me and continued the friendship over the years."
Quizzed by her co-hosts, the mom of three admitted that Leno, 63, was a good kisser but that she couldn't remember what the sex was like because "it was so long ago."