McIlroy, 25, landed the fourth major of his career by a stroke from Mickelson in thrilling style having traded blows and lost his one-shot overnight lead to the Americans and Sweden's Henrik Stenson for much of a rain-delayed final round.

The world number one's victory was his third in a row following wins at The Open and WGC-Bridgestone Invitational and this was his second PGA title in three years having also won the 2012 championship at Kiawah Island.

That was a record-breaking, eight-stroke procession but this was a bruising scrap and McIlroy was once again well able for the task.

He had looked jaded when he bogeyed the par-three fourth and par-fourth sixth as Mickelson got off to fast start with two birdies in his first three holes and Fowler overcame an early bogey at the second to sink three straight birdies.

A birdie at the seventh righted the ship but the Irishman was still two shots adrift of a three-way tie at 14 under between Mickelson, Fowler and Stenson as he made the turn for home.

The summer of 2014 is one I'll never forget!! #backtobackmajors pic.twitter.com/RGsHWSWWBA

— Rory McIlroy (@McIlroyRory) August 11, 2014

But then came the fireworks. McIlroy produced the shot of the day when he unleashed his three wood from the middle of the 10th fairway 281 yards from the pin, giving it a low run over the sodden ground to just seven feet and set up an eagle putt.

Had the rally begun? Of course it had, McIlroy moving to 14 under with the eagle three at a hole he had double bogeyed in the first round.

It jumped him to second place, one shot behind Fowler, with whom he had duelled a month ago at The Open and beaten by two strokes at Hoylake.

He was playing some wonderful golf but so were his three adversaries and McIlroy did not help his cause by missing birdie putts at both the 11th, from 10 feet, and the 12th, from seven.

So when he sent in his approach at the 13th to nine feet from the pin, there was an awful amount of pressure riding on the putt. Not that McIlroy had any doubts, sinking it with aplomb to move to 15 under and a fourway share of the lead.

Stenson and Fowler were the first to fall by the wayside, both coming unstuck at the long par-three 14th, where the Swede three-putted, missing his par putt from four feet, and the American followed two groups later after missing the green off the tee.

The protagonists were now battling the fading light as much as each other, with putts increasingly difficult to read and McIlroy looking on in disbelief when his birdie putt on the 15th came up short by a couple of feet.

Mickelson tried to bypass the greens completely as he nearly holed out with a chip from in front of a greenside bunker on 16, the ball lipping out of the hole but accelerating away to nine feet. The short-game genius missed the putt coming back and suddenly McIlroy was back in front, alone at the top of the leaderboard for the first time in 14 holes.

He parred the 16th as Stenson completed a final-round 66 to get to 14 under and then landed the killer blow at the par-four 17th, blasting his tee shot 318 yards into a fairway bunker and sticking his next shot from 150 yards to 11ft from the hole with a nine iron. There was still work to do from there but McIlroy, coolness personified, sank the birdie putt and went to the last with a two-shot lead over Fowler and Mickelson.

Conditions were now beyond gloomy and McIlroy was told to take his tee shot on the par-five 18th before his rivals ahead had played their second shots into the green. The hurry-up did him no favours, though as his drive flirted with the water on the right but landed in the rough.

His second shot bunkered to the left of the green before Mickelson attempted to level matters with a 70ft eagle chip from off the green. It came agonisingly close but rolled a foot past the post and the five-time major winner had to settle for bogey and hope for a slip-up from the Ulsterman.

McIlroy had three shots to eke out victory, though and while his bunker shot came up a little too short for his liking, he still had two putts to secure victory, sending the first from 34 feet to within inches. Job done and McIlroy's joy in the dark was illuminated by hundreds of flashbulbs.

McIlroy signed for a 68, three under par, to finish 16 under par and collect a cheque for $1.8m (€1.345m), emulating Tiger Woods in 2000 and 2006 by winning The Open, WGC-Bridgestone and PGA in consecutive appearances.

And at 25 years and 98 days McIlroy has become the fourth youngest player to win four major championships after “Young” Tom Morris (aged 21), Tiger Woods (24) and Jack Nicklaus (25).

It also ramps up the hype ahead of his assault on a career grand slam of major titles when he attempts to complete the set next April at The Masters in Augusta, Georgia. But thoughts of that can wait a while, McIlroy has earned himself a celebration.

Flying high with the team after my 4th Major victory!!! NYC here we come!! Thanks for all the support and well wishes pic.twitter.com/zK1q8xnwA8

— Rory McIlroy (@McIlroyRory) August 11, 2014

“The ball flight was probably around 30 feet lower than I intended and the line of the shot was probably around 15 yards left of where I intended,” McIlroy admitted. “It was lucky, it really was.

“You need a little bit of luck in major championships to win and that was my lucky break. I didn’t hit a very good shot there but it worked out well and I made eagle from it.”

“I want to thank Phil and Rickie for letting us tee off, if they hadn’t done that we might not have been able to get it done,” McIlroy said. “It showed a lot of sportsmanship and class from those two guys.”

McIlroy is the first player since Padraig Harrington in 2008 to win back-to-back majors, his victories in the Open Championship and US PGA also sandwiched by a first World Golf Championship win in the Bridgestone Invitational last week.

Lifting the Wanamaker Trophy for the second time also makes the Northern Irishman the third youngest player after Tiger Woods and Jack Nicklaus to win four majors, just one less than the total won by the late Seve Ballesteros and two behind Nick Faldo.

“It’s been just incredible,” McIlroy said. “I didn’t think in my wildest dreams I would have a summer like this. I just played the best golf in my life.”

McIlroy, who won the 2011 US Open and 2012 US PGA by eight shots and was six clear going into the final round of the Open, added: “I gutted it out today, it was a little different to my previous major wins. I think I showed a lot of guts to get the job done.”

Mickelson did not appear happy with the decision to allow McIlroy and Bernd Wiesberger to hit their approach shots to the 18th but added: “It didn’t affect the outcome, I think.

“It’s not what we normally do but it’s not a big deal. It’s a courteous thing to let the guys tee off in case they blow the horn. It gave everyone a chance to finish just in the nick of time.”

Fowler has now finished fifth, second, second and third in the year’s four majors but acknowledged he had let slip his first real chance to win.

“This is the first one that hurts,” Fowler said. “The performance in the majors is something I can be proud of but Rory has been a deserving champion every time. We will see if we can get one away from him at some point.”