Mahan earned a 4-up lead through 10 holes before McIlroy won the 11th with a chip-in eagle at the par 5. A birdie at the 14th allowed McIlroy to reduce the margin further, but he could get no closer.
With his second World Golf Championship title, Mahan moves to No. 9 in the world ranking. McIlroy remains second, having needed a win to surpass Luke Donald for No. 1.