'Today was one of those days,' said Harrington, who earned $990,000 for the win. 'You have to be patient and you know it doesn't matter how much you practice, you have to get out and play.'
'When I was making my run, especially when I started making all those birdies, I was thinking of 59,' said Harrington, who came from seven shots back to get into the playoff. 'I didn't think about winning the tournament.'
Joe Ogilvie was the third player to gain a spot in the playoff after his birdie try at the 72nd hole in regulation failed to find the bottom of the cup.
The trio returned to the difficult 18th to begin the extra session and Ogilvie virtually took himself out of the mix off the tee after his drive found a fairway bunker. Harrington followed and smacked his drive in the rough, while Singh hit a solid tee shot down the fairway.
Harrington's approach rolled up short of a greenside bunker. Ogilvie, with one foot in the sand, advanced his ball down the fairway before Singh dropped his second shot on the putting surface 13 feet from the hole.
Ogilvie missed the green with his third and his pitch ran past the flag. Harrington stayed alive after his third landed firmly within 5 feet of the hole. Singh then had a chance to secure his 26th career victory on the PGA Tour, but his birdie effort missed left.
'I hit a poor bunker shot, a poor third shot and a poor fourth shot,' Ogilvie said of the first playoff hole. 'That usually leads to bogey.'
Harrington then saved his par and the next time around found the fairway off the tee at the 18th, while Singh left his drive just to the right of Harrington's ball. Harrington hit another poor approach and watched his ball come up short and left of the green.
Singh followed suit and nestled his ball close to Harrington's. Harrington pitched his third within 6 feet and Singh calmly curled his shot well inside of 3 feet.
Harrington did his part and rolled in the par save, leaving Singh a short attempt to extend the playoff. The former No. 1 player in the world had a momentary lapse, however, and hung his head as his ball slid around the back of the hole.
'You can't miss putts like that in a playoff,' said Singh, who defeated Harrington in a playoff at the 2001 Malaysian Open on the European Tour. 'I shouldn't have missed that putt.'
Harrington was on fire early with a birdie at the first and three consecutive birdies starting at the par-4 fourth. He then played his tee shot to 12 feet for a birdie at the par-3 eighth to begin another impressive stretch.
The 33-year-old knocked his approach inside 7 feet for a birdie at the ninth and dropped his second shot within inches of the hole for a tap-in birdie at the par-4 10th. Harrington then drained a 6-foot putt for a birdie at the par-3 11th and sank another short putt for a birdie at the par-5 12th.
Harrington ran home a long birdie putt at the 13th to make it six in a row, but trouble was on the way. He missed the green en route to a bogey at the 14th and couldn't find the putting surface again for another bogey at the 15th.
'If I looked up and saw that I was three behind, I would have been down the last couple of holes,' said Harrington, who became the first Irishman to win on the PGA Tour. 'But knowing that I still had a chance of winning, it refocused me on the last couple of holes.'
He persisted, however, and tallied a birdie at the par-5 17th that was ultimately good enough to earn a spot in the playoff.
Singh birdied his first two holes, but gave a shot back at the par-3 third. He countered with a birdie at the fifth and an eagle at the par-5 sixth. At the par-3 eighth, Singh ran home a 25-footer to reach 11 under.
He added a birdie at the 10th and converted a 22-foot putt for a birdie at the par-5 12th. Singh held on down the stretch and two-putted for birdie at the 17th to join Harrington at minus-14.
Ogilvie, who finished second to Singh in New Orleans last year, picked up a pair of birdies on the front nine and a birdie at the 12th lifted him to 13 under.
The 30-year-old almost holed out his third shot at the par-5 17th, but he tapped in for birdie to move to 14 under. He had a chance to put the tournament away at the last, but was unable to convert.
Pat Perez finished alone in fourth place at 13-under-par 275 after a round of 70. David Toms shot a 67 to come in at 12-under-par 276.
Brett Wetterich and Geoff Ogilvy shared the lead heading into the final round, but both players experience disaster on the back nine Sunday. Wetterich triple bogeyed the par-4 13th on his way to a 73, while Ogilvy double bogeyed the last for a 73 of his own.
They were joined by Brad Faxon in a tie for sixth at 11-under-par 277.
Lucas Glover equaled the course record early in the day with a 63 to share ninth place at Jim Furyk at 10-under-par 278.