Singh won the title in 1998, then donned the green jacket with a win at the 2000 Masters. It was also Singh's fifth win of the season, virtually assuring himself Player of the Year honors.
'This makes my year,' said Singh, who pocketed $1,125,000 for the win. 'I played well at the Masters and I did not win. I played well the first two days of the U.S. Open, played well at the British, but this is it. I wanted one major again and it came at the right time.'
Singh, the third-round leader, struggled to a 4-over 76 in the final round. Leonard, who held a two-shot lead on the back nine Sunday, shot a 75 and DiMarco posted a 1-under 71. The trio finished regulation knotted at 8-under-par 280.
On the first extra hole, No. 10 at Whistling Straits, DiMarco and Leonard hit fairway-metals off the tee. Singh pulled driver and ran his tee ball just short of the green. DiMarco's approach took a bad bounce into the rough, while Leonard hit his second 20 feet from the hole on the back fringe.
Singh pitched to 6 feet. DiMarco's chip rolled 4 feet right of the hole and Leonard's birdie try ran 3 feet by. Singh sank his birdie putt and the two others made par to give Singh the advantage.
At the par-3 17th, Singh hit the shot of the tournament with a 3-iron that hit the slope in the center of the green. The ball ran down 5 feet from the cup. DiMarco's tee ball barley missed the putting surface some 80 feet short and Leonard landed in the rough on the right side.
Leonard pitched his second inside 4 feet and DiMarco chipped his birdie effort to 4 feet. Singh stepped up and missed the birdie putt left. All three made their shortish par putts and 18 was once again the stage for a great finale.
At the closing playoff hole, Singh held a one-shot lead and hit a 3-wood off the tee, while DiMarco and Leonard both found the fairway with drivers. Singh hit a utility wood 45 feet from the flag for his second.
Leonard and DiMarco both missed the putting surface, then DiMarco blasted a bunker shot 35 feet past the hole. Leonard chipped 5 feet by the flag so Singh needed to two-putt for the title.
Singh lagged his birdie putt a foot from the stick. He marked, then quickly tapped in for the par and the win.
Singh ditched the belly putter two weeks ago and won the Buick Open. This caps off that decision.
'Coming back from a belly putter and winning two in a row, I never thought at any stage that I was going to come back and putt so well so quickly and win golf tournaments,' Singh said.
Ernie Els and Chris Riley each bogeyed the last to shoot matching rounds of 1-over 73. They tied for fourth place at 7-under-par 281.
Reigning Masters champion Phil Mickelson never got anything going on Sunday and shot a 2-over 74. He shared sixth place with K.J. Choi (70) and Paul McGinley (69) at 6-under-par 282.
Tiger Woods, a two-time winner of the Wanamaker Trophy, posted a 1-over 73 and tied for 24th at minus-2. His majorless streak now dates back to the 2002 U.S. Open, a span of 10 tournaments.
'It's frustrating because I didn't win, simple as that,' said Woods. 'I felt like I was playing so well coming into this event, and I was. It's unfortunate that I just didn't continue that way, that first day when I played decent but putted so poorly.'
Woods may not have putted well, but Singh took 34 putts on Sunday and still managed to hoist the Wanamaker Trophy. Singh's birdie at the 10th in the playoff was his first of the day and his 76 was the highest final-round score by an eventual PGA Championship winner.
'I hung in there,' said Singh, who has now won the last eight times he has held a piece of the 54-hole lead. 'It looked ugly when you look at the score, but it was a good 4 over for me. I felt I played nicely. When you're in a playoff, you know you can't do worse than second. When Justin let me off at the last hole, that kind of gave me relief.'
Leonard bogeyed the 18th for the second time in as many days as the hole named, 'Dyeabolical' for course designer Pete Dye, lived up to its name on Sunday.
DiMarco had the best look all day when his 6-iron in regulation landed 12 feet from the hole. His birdie putt came up short, but that was the last good chance the final groups had.
Els missed a 6-foot par putt at the last that cost him a chance at the playoff. Riley, normally one of the steadiest players in the game with the flat stick, also missed a putt of less than 10 feet to fall out of the running.
When Leonard and Singh reached 18, Leonard, the 1997 British Open champion, missed the green short with a 5-iron from 204 yards out. Singh hit a 6-iron on the right side of the green, and missed his 35-footer short.
Leonard chipped 12 feet short of the stick. He missed the putt, then Singh holed his to force the three-man playoff, the first since John Mahaffey, Tom Watson and Jerry Pate in 1978.
'I just felt like it was a really good 5-iron,' said Leonard, referring to his approach on the 72nd hole. 'I was trying to hit it at the hole. It was a good club and I felt I was swinging the club nicely. When I hit the shot, I thought I just ended this golf tournament.'
Leonard built a two-shot lead Sunday afternoon, but DiMarco caught up with three birdies in a four-hole span from the ninth. When Leonard missed the green right at No. 10 and failed to convert the 20-footer for par, he and DiMarco were tied for the lead.
DiMarco fell down the leaderboard at 15 when his drive landed in the thick rough on the left side. He chipped eight feet by with his fourth shot, then holed the bogey putt to fall one behind Leonard.
Leonard drained an 18-foot birdie putt at 13 to move two clear of DiMarco. Both players dropped strokes on the second nine, DiMarco at 16, then Leonard at the 14th.
Leonard, who also lost a playoff at the 1999 British Open, had a chance at taking a three-shot lead, but missed a 7-footer for birdie at the 518-yard 15th.
Singh hung in despite hitting few fairways and even fewer putts. He parred every hole from eighth to the 14th, but bogeyed 15 when his approach went well left of the putting surface. Singh blasted 15 feet short and his par try lipped out of the hole.
Leonard now had a two-shot lead on the par-5 16th tee, but hit his 2-iron second shot into a bunker. His pitching-wedge landed on the fringe and his fourth stopped 5 feet from the hole. Leonard missed the par putt right, then parred 17.
Thanks to Leonard's mis-club on 18, Singh and DiMarco were given new life. Singh took advantage and moved his PGA Tour record to 4-1 in playoffs. Leonard fell to 0-4 in extra sessions and DiMarco moved to 1-1.
DiMarco may have lost his first big chance at a major title, but he picked up something else on Sunday. DiMarco's tie for second vaulted him to eighth on the American Ryder Cup points standing. The top 10 after Sunday automatically made the team.
'I'm proud of myself that I went out and got it done,' said DiMarco, who made the team for the first time. 'Once I made the par on 17 and hit my 6-iron on the green at 18, I looked at my caddie and said, 'that's good enough for sure.''
Riley's tie for fourth moved him to 10th, so he will be making his Ryder Cup debut. Fred Funk, who missed the cut, fell to ninth, but also made his first Ryder Cup team.
Steve Flesch and Jay Haas were ninth and 10th on the points table, respectively, but got bumped out this week. They will have to wait to see if U.S. captain Hal Sutton selects them as captain's picks Monday morning. So will Leonard. He had to win to move into the top 10.