Watson won for the fifth time in his career, and he had to earn it. With no margin for error over the closing holes, he managed to get out of a deep fairway bunker on the 15th hole to the front edge of the green. He smartly played to the middle of the green on the par-3 16th hole for a par. Facing the uphill tee shot on the 18th, he blasted his drive down the middle of the fairway and hit wedge into 15 feet to the right of the pin.
Johnson, meanwhile, now has finished among the top six in all four tournaments this season — including a win in Shanghai and consecutive runner-up finishes. His other start was at Kapalua, where he tied for sixth.
"I had a chance there on the back nine, I just didn't have good looks on 16, 17 and 18 to give myself a chance," he said. "I was still right there. That's all you can ask for."
McGirt, who had a two-shot lead to start the final round as he tried to win for the first time, opened with a birdie and stalled after that. And on yet another gorgeous day at Riviera, this was not a day to stall.
The pack behind took aim and made birdies, particularly Watson.
Watson hit a perfect tee shot on the par-3 fourth that rode the slope to 15 feet for birdie. He holed a bunker shot from left of the sixth green for birdie. And a 15-footer on the eighth gave Watson his fifth birdie of the round, and the outright lead. He never gave it back.
McGirt closed with a 73 and tied for sixth.
Jordan Spieth, one shot out of the lead through seven holes, had four bogeys on the back nine and shot 71 to tie for 12th. Jimmy Walker, a three-time winner this season, closed with a 73 and tied for 20th.
"My first win since the Masters," Watson said. "You never know when your last win is going to be. My last win could have been the Masters, which would have been a great way to go out. But winning here at Northern Trust is nice. I never felt down that I haven't won yet, but just kept plugging along. And somehow, it fell in my lap today."