SHANGHAI, November 3, 2013 (AFP) - Johnson finished on a tournament record 24-under par of 264, three ahead of England's Ian Poulter and four ahead of Northern Ireland's Graeme McDowell after a pulsating final day's play captivated huge galleries at Sheshan Golf Club.
The leading trio all shot final rounds of six-under par 66, going head-to-head in a display of golf of the highest quality.
McDowell, a former US Open champion who is second in the European Tour's Race to Dubai standings, and Poulter got off to a perfect start as they both birdied the first two holes.
Johnson, starting the day three ahead at 18 under par, dropped a shot by three-putting the first.
When McDowell added a third birdie in a row at the third the American found himself in a three-way tie for the lead at 17-under.
"Those first few holes weren't fun," admitted Johnson afterwards. "I knew they were going to come out swinging."
The lead ebbed and flowed with first McDowell, courtesy of a birdie at the seventh, edging in front. But he was soon joined again by Poulter and Johnson who both birdied the eighth. Johnson got his nose in front again with another birdie at nine.
The par-three 12th looked as if it might be a turning point as first Poulter sank a 25-footer from off the green and then McDowell followed him in from half that distance in the fringe to put the European Ryder Cup duo back out in front.
But the big-hitting Johnson kept his cool, ripping a monster drive some 350 yards down the 411-yard 13th before floating in the deftest of lob wedges from 70 yards to a pin cut tight to the sloping front edge. The putt, almost a formality, dropped and the trio were back all square.
All three thrilled the galleries by rattling home a succession of birdie putts at the par-five 14th and Johnson said that was the turning point.
"I drove in the rough and hit it just short of the green. I then hit a very poor wedge and left it 20-foot short. Graeme made about a 50-footer and for me to make the next one, especially since Poulter was already in there for birdie, I think that was the big turning point."
Poulter bogeyed 15 after finding a bunker left of the green and Johnson pounced with the killer blow at the short par-four 16th.
With McDowell and Poulter both inside 12 feet with their second shots, Johnson -- who was just 20 yards off the front edge of the green with his drive -- chipped in for an eagle two.
"If I had to place my tee shot off the tee to leave the perfect pitch shot, that is where I would have put it," said Johnson.
It seemed to knock the resistance out of the European pair. Although Poulter holed his putt for birdie, Johnson had a two-shot cushion.
He then put his tee shot to within eight feet at the 17th and rolled home another birdie to seal the title.
"This is my biggest win," said Johnson who picked up $1,400,000 that will catapult him to 12th in the world golf rankings from 23rd.
It is his first World Golf Championships win in his 17th appearance: his previous best was second in the 2011 WGC-Cadillac Championship.
Americans have now won all four WGC events in 2013. Matt Kuchar won the Accenture Matchplay and Tiger Woods took both the Cadillac Championship and Bridgestone Invitational. Both players decided not to compete in Shanghai this week.
Leading scores on Sunday after the final round of the $8.5 million World Golf Championship-HSBC Champions at Sheshan Golf Club, Shanghai (par 72):
264 - Dustin Johnson (USA) 69-63-66-66
267 - Ian Poulter (ENG) 71-67-63-66
268 - Graeme McDowell (NIR) 69-69-64-66
270 -Sergio Garcia (ESP) 70-68-69-63
272 - Justin Rose (ENG) 68-71-65-68
273 - Rory McIlroy (NIR) 65-72-67-69, Graham Delaet (CAN) 71-68-65-69
274 - Martin Kaymer (GER) 70-74-62-68, Bubba Watson (USA) 68-69-69-68, Jamie Donaldson (WAL) 67-74-66-67
275 - Keegan Bradley (USA) 71-68-68-68, Ernie Els (RSA) 69-69-71-66, Boo Weekley (USA) 70-67-69-69
276 - Phil Mickelson (USA) 71-68-72-65
277 - Liang Wenchong (CHN) 72-67-72-66, Louis Oosthuizen (RSA) 70-70-70-67
278 - Jordan Spieth (USA) 68-71-70-69
279 - Tommy Fleetwood (ENG) 68-70-69-72, Jin Jeong (KOR) 70-69-71-69
280 - Paul Casey (ENG) 69-73-69-69
Selected:
282 - Luke Donald (ENG) 70-71-70-71, Henrik Stenson (SWE) 74-76-67-65
287 - Lee Westwood (ENG) 71-73-68-75