Dustin Johnson finished with a 21-under 263 total to win this event at the Club de Golf Chapultepec for the second time in three years.
Dustin Johnson captured his 20th US PGA Tour title on Sunday, firing a five-under par 66 to beat Rory McIlroy by five strokes in the final round of the WGC-Mexico Championship.
The 34-year-old American finished with a 21-under 263 total to win this event at the Club de Golf Chapultepec for the second time in three years.
"It clicked. I hit it really well this week," Johnson said. "It means a lot, to get 20 wins out here is very difficult. To do it before I turn 35 is pretty incredible.
"This is a big win for me. It gives me a lot of confidence for the rest of the year. I feel like the game is in good form right now."
McIlroy closed with a 67 thanks to a sizzling back nine -- six birdies in seven holes before a bogey at 18 -- but it wasn't enough to catch red-hot Johnson, who posted five of his six birdies on the back nine.
Johnson has won at least one US PGA event in 12th consecutive years, with his 20th career win coming in his 246th start.
Englishmen Paul Casey and Ian Poulter shared third on 273, shooting 65 and 68 respectively in the fourth round, with Thailand's Kiradech Aphibarnrat, who fired 68 to join the pack 10 off Johnson's pace.
Sergio Garcia, Patrick Cantlay and Cameron Smith shot identical 70s and shared sixth.
Justin Thomas had the round of the day with a nine-under 62 which left him ninth. He had four consecutive birdies on numbers 10-13 and fashioned nine birdies over his first dozen holes.
In was another stroke back to Tiger Woods, who closed out his week with a 69 and tied for 10th with fellow Americans Keegan Bradley and David Lipsky and Dutchman Joost Luiten.
Johnson got off to a tentative start with a bogey on the par-three third hole before making birdie on the par-five sixth.
"I felt some nerves at the beginning of the day but then I made some really good par saves early in the round and it gave me some momentum," Johnson said.
'Easy game for him'
He kicked it into high gear on the back nine with back-to-back birdies on the 10th and 11th holes and then strung together three birdies in a row beginning at the par-four 14th.
"I just think that this place, it definitely favours someone who's driving it well," Thomas said. "But as good as DJ's wedge game is, it's a pretty easy game for him sometimes."
McIlroy failed to take advantage of Johnson's sluggish start with two bogeys on the front nine at the fourth and sixth holes.
McIlroy got hot on the backside, making six birdies in a seven-hole stretch before closing with another bogey on the par-four 18th.
"No contest," said McIlroy. "Especially here. His drives are going 380 down the middle of the fairway and he's holing most of the putts he looks at."
Woods, a seven-time winner of the tournament, struggled on the greens for the second consecutive day.
The 14-time major champion missed short birdie putts on the par-5 15 and the par-4 16th hole. Woods now takes a week off before returning for back-to-back events at the Bay Hill and The Players.
Reigning Masters champion Patrick Reed shot a final round 73 while playing in the final group. He finished tied for 14th.