McIlroy closed with his second straight 4-under 66 to end at 15-under-par 265. The win will move the Open Champion back to the No. 1 spot in the world rankings on Monday.
Third-round leader Sergio Garcia was unable to get anything going in the final round. He carded a 1-over 71 and ended alone in second place at minus-13 at Firestone Country Club.
Adam Scott, the 2011 champion, bogeyed three in a row on the back nine to fall out of the top five. He managed a 1-over 71, which left him tied for eighth at 7-under-par 273. By finishing outside the top five, Scott will lose his spot atop the world golf rankings on Monday.