American Dustin Johnson capitalised on a collapse from runaway leader J.B. Holmes to take out the WGC-Cadillac Championship by one stroke in Miami on Sunday.

Johnson - winner of the WGC-HSBC Championship in 2013 - carded a three-under-par 69 to finish at nine under through 72 holes at Trump National Doral.

Holmes was in a commanding position from the outset in Miami but threw away a five-shot lead on the final day, shooting a three-over-par 75 for the runners-up cheque.

Dual Masters champion Bubba Watson (71) was a shot further back after holing three bogeys on the back nine.

"It means everything," said Johnson, who sealed victory with a par putt at the 18th hole.

"It's been a tough road but I played really good. It feels awesome to get that win. I played great today.

"I knew I was really good. I knew there was something I was missing that could make me great.

"I was working hard on that, and I think it's showing right now. ... It's tough. I'm so excited right now, I can hardly talk. It feels great. This one definitely, by far, is the best one."

No one expected Johnson - in his fifth start since returning from a six-month leave of absence - to win the lucrative PGA Tour event, with Holmes' victory seeming a mere formality.

Holmes, who matched Johnson's hole-in-one on the par-three fourth hole on Saturday, was five shots clear at the start of play but three bogeys on the front nine gave his rivals a glimmer of hope.

And Johnson took full advantage, going bogey-free on the back nine, with two birdies lifting the nine-time PGA Tour champion to a confidence-boosting win, having missed the cut at The Honda Classic last week.

Australian Adam Scott (71) - in his first tournament since switching to a shorter putter - finished fourth at four under alongside Swede Henrik Stenson (72).

Louis Oosthuizen (73) of South Africa was in sole possession of sixth at three under.

A shot further back were Americans Webb Simpson (73) and Bill Haas (74).

Meanwhile, world number one Rory McIlroy (72) ended the tournament at one under alongside Kevin Na (71) and Ryan Moore (76).