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."
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