In a nail-biter of a final round that stirred memories of Jean Van de Velde's collapse eight years ago, Harrington lost a one-shot lead on the 72nd hole by hitting into the Barry Burn -- not once, but twice -- on his way to double bogey.
Given a one-shot lead, Sergio Garcia gave it right back when he couldn't make par from a bunker on the last hole.
Harrington staked himself to a two-shot lead in the four-hole playoff. Instead of the bravado Van de Velde showed in 1999, the Irishman played it safe off the 18th tee in the playoff with an iron off the tee and laying up short of Barry Burn.
As the engraver began stenciling the Irishman's name into the claret jug, Garcia made a gutsy play by smoking a 6-iron from 203 yards out of the rough to birdie range. His 25-foot putt skimmed the edge of the cup, leaving Harrington a 3-footer for the win.
'It's going to take a long time for it to sink in,' Harrington said, the first Irishman to win the British Open in 60 years. 'I know it was only a short putt, but the emotions of it ... I couldn't believe it as it was rolling in from right in the middle of the hole, and I'm thinking, 'The Open champion.' A huge amount of it was genuine shock.'
Blame that on Carnoustie.
Garcia was devastated, blowing a three-shot lead going into the final round only to be handed a gift he couldn't take. He blasted out to 10 feet from the bunker on the 18th, and still doesn't know how the ball stayed out of the cup. It dipped slightly and spun away, forcing the third British Open playoff in the last six years.
'To tell you the truth, I don't feel like I did anything wrong,' said Garcia, who closed with a 73 and was 1 over in the playoff. 'I really didn't miss a shot in the playoff. I hit unbelievable putts. They just didn't go in.'
Harrington, who closed with a 4-under 67 to make up a six-shot deficit, played the four playoff holes in even par, seizing control with a 7-iron into 8 feet for birdie on the first hole as Garcia made bogey from a bunker.
As it all wrapped up, a rainbow stretched over the course by the North Sea, capping another magical day on perhaps the toughest links in golf.
Like the last Open at Carnoustie, there was chaos in the end.
Only this time, it involved more than one player.
Van de Velde self-destructed on his own in 1999, blowing a three-shot lead on the final hole by hitting one shot off a tiny rail in the grandstand, another into the burn, another in the bunker.
Eight years later, the bad luck belonged to Andres Romero of Argentina, who looked like another unlikely champion at Carnoustie until his 2-iron from the rough on the 17th hole ricocheted off the stone wall of Barry Burn and across the 18th fairway, out-of-bounds.
The bizarre bounce went to Harrington, whose tee shot on the 18th hole hopped along a bridge over the burn until it went between two rails and into the winding stream. He also chunked his next shot into the burn, then made a clutch putt from 5 feet for double bogey.
That left only bad timing for Garcia.
This just wasn't his day.
It was his third time to play in the final group of a major, this time with Tiger Woods out of the picture early. But the 27-year-old Spaniard couldn't buy a putt, and he couldn't get a break.
Two shots down with two holes to go in the playoff, and with Harrington in trouble right of the par-3 16th green, Garcia watched in disbelief as his tee shot smacked the base of the pin and went 18 feet away.
He missed the birdie putt. He missed a lot of putts in the final round, which remains the weakness in his game.
'Every time I get in this position, I never have any room for error,' Garcia said. 'I should write a book on how not to miss a shot and not win a playoff.'
But he missed several shots, muttering to himself, pleading on one occasion, 'Please, please, please my God, please.'
He closed with a 73, joining Harrington in the playoff at 7-under 277. The winning score was 13 shots lower than it was the last time at Carnoustie, but everything else -- especially the final holes -- was eerily similar.
Almost lost in the crazy finish was Harrington, who ended Europe's eight-year drought without a major.
It also ended the reign of Woods at golf's oldest championship. Trying to become the first player in 51 years to win the claret jug three straight times, he was never a factor. He made two early birdies, but finished with a 70, shots behind in a tie for 12th.
'I wasn't as sharp as I needed to be,' Woods said.
Romero shot a 67 in the final round and was the only player to shoot par or better all four days. He had a 34 on the back nine, despite two double bogeys that kept him from joining Angel Cabrera as a major champion from Argentina.
He had more to moan than Garcia. Romero's par putt from 12 feet that ultimately would have put him in the playoff hit the back of the cup and spun away, leaving him at 6-under 278.
'The pressure certainly caught up with me,' Romero said.
The bounce off the burn was so bizarre that it took a moment to figure out where it went, much like Van de Velde's approach that caromed off a tiny rail on the grandstand left of the 18th green in 1999.
'I did it on 17, not 18,' Romero said when asked if would be linked to the Frenchman's follies. 'But I could be put into that category by some. I certainly wasn't thinking about Jean Van de Velde at that moment.'
Richard Green of Australia matched the British Open record at Carnoustie with a 64 and finished at 279 with Ernie Els (69). Hunter Mahan made the cut on the number and tied for sixth with a 69-65 weekend.
Harrington was equally theatrical.
He got back in the game with a brilliant approach on the par-5 that trickled onto the green to 12 feet away for eagle, a turnaround when Romero made his double bogey.
Harrington came to final hole with a one-shot lead, but he knew his drive was in trouble when it headed right toward the hairpin shape of the Barry Burn. He almost got the break of a lifetime.
It dropped into the water, and Garcia knew exactly what was going on. He was coming up the 17th fairway when Harrington was figuring out how to play his third shot to the 18th. They passed each other on the bridge, and Garcia smiled.
Harrington's next shot might have been worse. It was a 5-iron he hit so heavy that it bounced into the burn in front of the green, just like Van de Velde's did. There was no thought of removing his shoes and stepping into the burn. Instead, he quickly took his drop, hit a nifty pitch to 5 feet behind the hole and made a putt that gave him a sliver of hope.
That gave him a 67, and Garcia gave him some help.