Dustin Johnson

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.

Johnson played his last seven holes knowing he had to review a possible rules infringement after the round.

He shot 68 to finish five under but was then penalised a shot for making his ball move on the fifth green, despite being initially absolved of wrongdoing.

McIlroy called organisers "amateur" and Spieth said the decision was "a joke".

Johnson's score was amended to four under, but he still finished three shots ahead of Irishman Shane Lowry and American duo Jim Furyk and Scott Piercy at Oakmont.