With the victory, American Johnson jumps from third to first in the world rankings, supplanting Australian Jason Day.
Johnson started the final round with a five-shot lead and was never headed after birdies at the first two holes en route to a closing 71 on the storied Riviera course.
Despite two late bogeys, he finished at 17-under-par 267, with Thomas Pieters (63) of Belgium and American Scott Brown (67) a distant second on 12-under in the event formerly named the Los Angeles Open.
