Now television viewers can see him going up against a real machine.
On Monday, "Jeopardy!" will begin airing two matches spread over three days between Jennings, Brad Rutter (the show's all-time money-winner with $3.25 million) and a computer named Watson developed by IBM scientists.
The scientists hope Watson, named for IBM founder Thomas J. Watson, is more than just a game-show curiosity. They see it as a real advance in the field of artificial intelligence with potential implications for business and medicine over the years to come.
They began developing Watson in 2005, about the same time Jennings was making his record run.
The challenge was to develop something more advanced than Deep Blue, another IBM machine that defeated chess champion Garry Kasparov in 1997. Watson required more nuance, the ability to understand puns and word games that are often part of the "Jeopardy!" experience.
Jennings, a former programmer who knows the limitations of computers, questioned whether the scientists would succeed.
"I was dubious," he said. "Some of that is ego. We don't like to think that our own particular niches or talents can be taken by a computer."
Watson had to take the same test that prospective human "Jeopardy!" contestants face, said Harry Friedman, the show's veteran executive producer. It passed. During fine-tuning, Watson participated in about 100 dry-run games with past "Jeopardy!" champions and won the majority of them, he said.
"When IBM came to us three and a half years ago, there was a process through which we began to understand the importance of the technology that they were developing," Friedman said. "That's what we found attractive - to be part of something that is a game-changer."
He asked IBM to help prepare material explaining Watson's development and the practical implications of the technology, which will be featured on the broadcasts next week.
There's already a book written about it - "Final Jeopardy: Man vs. Machine and the Quest to Know Everything" by Stephen Baker - ready to come out shortly after the broadcasts are aired. Everyone is trying to keep the secret of who won, to build interest in the shows.
"Going in I felt a little intimidated," Jennings said. "I felt like an underdog. I knew that this was an opponent who could not be intimidated."
During his record run, Jennings saw opponents who were psyched out even before going on stage with him because they had heard about his winning streak.
That certainly wasn't a problem with Watson, which is represented onstage with a plasma screen and artificial voice.
"Not only does it not care about my resume or Brad's, it can't get discouraged if it gets one wrong," he said. "It can't get cocky if it gets one right. It always makes the best possible play at every single point. There was no consideration for ego or psyche."
It was like playing against someone with the best conceivable reflexes, he said.
"I frankly thought it wouldn't happen," he said. "Once I saw Watson compete at a high level, I was very impressed. This is not just a little incremental step. This is a huge breakthrough."