A stroke in front of the chasing pack overnight, the Northern Irish world number one lost the lead but regained control after the turn, signing off with a three-under-par 68 at Valhalla Golf Club for a 16-under total of 268.
Phil Mickelson, the 2005 winner, birdied 18 to finish alone in second with a closing 66, with Swede Henrik Stenson (66) and American Rickie Fowler (68) a stroke further back in third.
“I didn’t think in my wildest dreams I’d have a summer like this,” British Open champion McIlroy told CBS Sports after being presented with the coveted Wanamaker Trophy, which he hoisted high in celebration.
“I’ve just played the best golf of my life and just really gutted it out today. It was a little different from the previous major wins that I’ve had and I think I showed a lot of guts out there today to get this job done.”
Trailing by three at one point, McIlroy got within a stroke of the lead with a spectacular eagle at the par-five 10th where he hit his second shot from 281 yards to seven feet, before effectively sealing the title with birdies at the 13th and 17th.
In gathering gloom at Valhalla, where play was suspended for just under two hours earlier in the day due to water-logged conditions, McIlroy parred the last to win his third consecutive title on the PGA Tour, and his second major this year.
The 25-year-old, who won the 2012 PGA Championship at Kiawah by a record eight shots, became the fourth youngest player to land four majors, with only Tom Morris Jr., Jack Nicklaus and Tiger Woods ahead of him.
Mickelson, who had been bidding for a sixth major title, recorded his first top-10 finish on the 2013-14 PGA Tour after a disappointing season.
“It was good for me to get back in the mix and to feel the pressure and get in the thick of it,” said the American left-hander. “I kind of ran out of steam there after 12, wasn’t able to get a couple of birdies coming in like I needed to.”
With birdies abundant on a receptive, rain-softened layout, the galleries were treated to a breathtaking display of shot-making in the final round.
RACE AGAINST DARKNESS
However, the late starters faced a race against darkness in their bid to finish the 96th PGA Championship on Sunday.
Play was halted just before noon as heavy rain swept across the par-71 Valhalla layout, leaving rivers of water on several fairways and pools covering many greens.
Roughly an inch of rain had saturated the course by the time play resumed at 2:44 p.m. (7.44 p.m. BST), and with dusk fast approaching a Monday finish seemed likely.
One ahead going into the final round, McIlroy ran up a three-putt bogey at the par-three third to slip to 12 under, level with Mickelson and Austrian Bernd Wiesberger.
Moments later it became a five-way tie at the top when Fowler birdied the fourth and Stenson the fifth.
Fowler seized the lead with a spectacular chip-in for birdie from just off the right edge of the green at the fifth, but was caught soon after by Mickelson and Stenson.
An up-and-down birdie at the seventh put Fowler one ahead again but the lead did not last long as Stenson soon joined him, draining a 27-footer at the ninth.
McIlroy, who had bogeyed the sixth after finding a bunker with his approach, picked up his first shot of the day at the seventh, hitting a delicate chip from beside a bunker to four feet and sinking the putt.
Out in one-over par 36, McIlroy eagled the 10th, narrowly missed birdie putts at 11 and 12 before sinking a five-footer at the 13th to join Mickelson and Fowler in a tie for the lead at 15 under.
Fowler dropped back with a bogey at 14, and Mickelson dropped his only shot of the day at the 16th before McIlroy effectively made the title safe when he rolled in a 10-foot birdie putt on the 17th green. – Reuters