USPGA champion Keegan Bradley carded a level par 71 to win the 36-hole PGA Grand Slam by one shot in Bermuda.
He began the final day of the annual event for the year's four major winners tied with Rory McIlroy at four under.
But McIlroy fell away with three successive bogeys from the seventh and finished third at level par after a 75.
Masters champion Charl Schwartzel, four behind overnight, birdied five holes in a row in a fine 65 to seal second, but Darren Clarke was nine over after a 74.
McIlroy, who birdied three of the last four holes in the opening round, failed to pick up a shot on day two but found himself three clear after six opening pars, with Bradley dropping three strokes in that time.
But by the turn the young Northern Irishman had dropped back and it was Schwartzel who was tied for the lead with Bradley, after holing from the bunker at the ninth to record his fifth successive birdie.
However Bradley, who bogeyed the opening hole after his approach shot spun back into the water, pitched to within four feet to birdie the elevated 10th and claim a lead that he would not relinquish again.
The 22-year-old American, who was overlooked in favour of Tiger Woods and Bill Haas as a wildcard for the Presidents Cup by USA captain Fred Couples, collected $600,000 for his victory, with Schwartzel picking up $300,00, McIlroy $250,00 and Clarke $200,000.
After making a five-foot putt for a par to seal the title, Bradley said: "It feels great. It was a lot more intense than I thought it was going to be - coming down the stretch it felt like any other tournament that I played in.
"To beat some of the best players in the world makes it even more special. It's a really nice way to kind of cap off the year, although I've still got a lot of golf to play.
"If you would have told me at the beginning of the year I would have won the Grand Slam of Golf tournament I probably would have laughed at you. It all kind of seems like a dream right now."
McIlroy said of his errant round: "I didn't quite have control of the golf ball like I did yesterday. The wind was tough, I just didn't do enough when I needed to."
Meanwhile Clarke, who dropped five shots in seven holes from the ninth to cancel out two early birdies, said: "Obviously I haven't played well the past two days. I've just been making a few changes and getting way too steep on my backswing.
"A shame I played so poorly this week - I'm looking forward to a very bad hangover in the morning."