Argentina's Tano Goya and Rafael Echenique were at 11-under 61, with Japan another shot behind. The United States pair of Nick Watney and John Merrick was tied for 15th at 5-under 67 after four birdies by Watney.
Defending champion Sweden was tied with Italy, Canada and South Korea six strokes off the lead in the $5.5 million tournament at the Mission Hills Golf Club. Friday's second round will be alternate-shot, as will Sunday, while Saturday will be better-ball again.
Aiming to win Ireland's third World Cup, McDowell and McIlroy each birdied six holes on the 7,251-yard course. McIlroy eagled the par-5 15th hole.
"We got off to a great start today and we birdied the holes that we knew we had to on the front nine," McIlroy said. "We had a chance of two birdies most holes and most of the time we took one of them."
England's Ian Poulter and Ross Fisher finished eight strokes back with a 6-under 66.
