Demsey, a former NCAA champion and amateur teammate of Tiger Woods and Phil Mickelson, closed with an 8-under 64 in the sixth and final round of Q-school to earn one of 26 full-exempt cards next year in the big leagues.
Frank Lickliter, who opened with consecutive 62s, completed a wire-to-wire week at Orange County National and finished as the medalist at 29-under 403. He won by four shots over Brett Rumford of Australia.
But the day belonged to Demsey, 35, who played bogey-free on the Crooked Cat course and was carried along by an amazing calm.
'I was just playing without any fear today,' Demsey said. 'It feels good to be calm under the most stressful situations. I really was fine with whatever happened. I came here to get my card, but it's not life or death. I have things in perspective.'
Demsey's only other year on the PGA TOUR was in 1997, when he made the cut in only nine of 27 events as a rookie. Back injuries slowed his career on the Nationwide Tour, but the real jolt came in 2002 when he felt constant pressure in his left sinus. At the end of the year, doctors found a tumor behind his left sinus going into his brain.
It took two operations to remove the tumor, but Demsey got another scare earlier this year when he discovered the tumor has returned. Doctors believe it is benign, and he will have an MRI next week to decide whether to treat it with radiation.
'They don't think it's going to be too much recovery time, probably just a week or so,' Demsey said. 'Hopefully, I can get it done in the next week or two.'
He doesn't have a lot of time, and for that he was thrilled.
Demsey was at 19-under 413 and finished eighth at Q-school, which means he likely will start his year at the Sony Open in Honolulu, just five weeks away.
Full-exempt cards were awarded to the top 25 and ties, and Demsey was in a tie for 31st starting the final round. He birdied his first hole and kept right on going, and when his 30-foot birdie putt on the 14th found the bottom of the cup, he knew he was close. Then came his seventh birdie of the day, on the 16th, and Demsey knew he was a lock.
The cutoff for cards came at 14-under 418.
Unlike previous years, there weren't too many horror stories, although Greg Chalmers and Miguel Angel Carballo each bogeyed the 18th hole and missed a card by one shot. John Merrick and Bob Sowards each birdied the final hole to earn their cards.
Chalmers and Carballo were among 53 players who earned cards on the Nationwide Tour.
Among those who left Q-school with no status was Colt Knost, the U.S. Amateur and U.S. Public Links Amateur champion. He turned down his automatic invitation to the Masters by turning pro, but was two shots shy of earning Nationwide status.
Casey Wittenberg, low amateur in the 2004 Masters, also missed Nationwide status by one shot.
Also failing to earn cards were former Ryder Cup players Chris Riley and Steve Pate, and Ted Purdy, who was 125th on PGA TOUR money list going into the final event of the year and missed the cut at Disney.
A couple of past champions on the PGA TOUR earned their cards by finishing in the top 25, including Duffy Waldorf, who closed with a 66, and Carlos Franco of Paraguay, who shot 70.
Not many felt quite as satisfied as Demsey, especially considering his long road back.
Demsey won the NCAA title at Arizona State in 1993, the year after Mickelson graduated. He was teammates with Woods at the World Amateur Team Championship in 1994, and played alongside Justin Leonard in the Walker Cup a year earlier.
He will join them again on the PGA TOUR, and he said he won't take a tour card for granted.
'When I got my card the first time when I was 24, I just thought it was easy,' Demsey said. 'I was pretty naive. Now I appreciate it more, and hopefully, I'll be better prepared.'