McIlroy became the first player since Tiger Woods in 2009 to win in consecutive weeks on the PGA Tour, and with his sixth career win, he joined Woods and Jack Nicklaus as the only players to win at least six times at age 23.
"I didn't think everything would happen so quickly, but I'm on a good run at the minute and I want to keep it going as long as I can," McIlroy said.
Mickelson and Westwood tried to chase him down on the back nine at Crooked Stick, only to mistakes when they couldn't afford any.
Westwood, who lost to McIlroy in the semifinals of the Match Play Championship in February, caught him with a birdie on the par-3 13th. But the weak area of his game showed up at the wrong time a poor chip on the 14th for bogey, another pedestrian chip on the par-5 15th that led to par. He wound up with a 69.
"I played with him when he was 13, and you could see it then," Westwood said. "He's just maturing all the time, as he will do. And he's a very, very good player.
Mickelson made back-to-back birdies late in the round to get within two shots of the lead, but he badly missed a 3-foot par putt on the 17th to fall three shots behind. He closed with a 70.
In Williamsburg, Va., Paula Creamer and Jiyai Shin played until dark, and still couldn't break their deadlock in the Kingsmill Championship. After they each parred the par-4 18th hole eight times in a playoff Sunday, darkness forced the suspension of play for the day. They will resume Monday morning on the par-4 16th hole.