Adam Scott fired a course-record 64 to storm past Justin Rose and claim victory at the PGA Grand Slam of Golf in Bermuda.
Rose had led by a single stroke with three holes to play but he had no answer for Scott in the closing stages as the Australian won the 36-hole tournament - which gathers the season's four major champions - by two shots on eight under.
The Masters winner took control on the penultimate hole when his six-iron landed within inches to allow him a tap-in eagle three.
US Open champion Rose had given up his single-shot lead at the previous hole, forced to take a drop after a wayward tee shot, before par on the 17th left him two shots adrift as Scott accelerated.
Scott's win means that he was able to add the winner's pink jacket to the green one he dramatically secured following a play-off against Angel Cabrera at Augusta in April.
Harrington was the defending champion after he stepped in for Ernie Els last year.
Rose admitted he thought he might claim victory when he had opened up a four-stroke lead on the front nine, but was left to praise Scott for his finish.
"At that point I felt confident I was going to win but I was just going about playing my game," he said.
"I didn't have my best game all week really and I'd have been surprised (to win).
"I'd have really been pleased if I had gone ahead and won because I didn't really feel like I had my best stuff.
"I struggled and hit one bad shot in the hazard on 16; and I didn't hit a lot of great shots on the back nine - I was hanging on.
"I started with a two-shot lead and somebody had to shoot well to beat me.
"When you're in that position you want to close it out, but when somebody finishes it out like that, you have to tip your hat and say you were beaten rather than losing."