Not in the four-man event - meant to be a battle between the season's major winners - until Ernie Els pulled out injured last Saturday, and only there because Rory McIlroy and first two reserves Graeme McDowell and Tiger Woods all turned it down, the 41-year-old Irishman had rounds of 66 and 67 to beat US Open champion Webb Simpson by one and both Masters champion Bubba Watson and last year's winner Keegan Bradley by six.
He joins 1991 champion Ian Woosnam as the only European winners of the title, and it was a case of third time lucky after losing play-offs to Angel Cabrera and Jim Furyk in 2007 and 2008.
Two ahead overnight, Harrington went to the turn in 34 with birdies at the fifth and eighth, but with eight holes to play was only one in front of both Simpson and Watson.
Then came a hat-trick of birdies from the 11th, and although Simpson narrowed the gap by picking up shots at the 14th and 17th, the Dubliner was able to bogey the last and still triumph on nine under par.
Harrington's last victory came in Malaysia 24 months ago, but he has not tasted success on the European or PGA Tours since 2008 and is down at 57th in the world.
After three-putting the 18th Harrington, who switched from the BMW Masters in Shanghai when the invitation came, said: "It was always the right decision no matter what.
"You've got to give yourself the chance - it's a fantastic tournament and even if I finished fourth I'd be quite confident that I made the right decision.
"It is a bonus to come and win, no doubt about it, and it was unfinished business for me having lost in two play-offs.
"I believe I'm playing really good golf. I see a lot of good things happening and I do believe that I'm turning the corner into a peak. What those peaks are, we'll wait and see."
Simpson said after his bogey-free 65: "He's playing well and made the putts you've got to make, but I'm really encouraged with how I played.
"My weakness this year has been when I'm in between clubs and I try to smooth one. My body kind of shuts down and doesn't work as well - it's just an eye-opener that I've got a lot of work to do in the off?season.
"I wish I could have putt a little more pressure on Padraig (at the last), but he'd probably have two?putted if he had to."