Northern Ireland's Rory McIlroy reduced Johnson's lead to two early on and sunk six birdies on the back nine as he shot a four-under 67 to finish 16 under.

World number three Johnson, four ahead overnight, responded with five birdies in seven holes to post a 66.

England's Paul Casey and Ian Poulter were tied in third place on 11 under.

Casey moved up 10 places on the leaderboard with a final round of 65, with four birdies on the opening six holes. He and Poulter shared third place with Thai Kiradech Aphibarnrat.

Tiger Woods carded a final-round 69 with three birdies to finish on eight under in joint 10th place.

Johnson, 34, becomes only the fifth player in the last 50 years to reach 20 PGA Tour wins before the age of 35, putting him in the company of only Woods, Phil Mickelson, Johnny Miller and Tom Watson.

"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."

Defending champion Mickelson, who opened with a 79, rolled in a 12-foot birdie putt at his final hole for a 68 and a level par final total.

Round of the day belonged to Justin Thomas, who carded a nine-under 62 to finish nine under overall.