On the opening hole Mahan flubbed a chip and ended up with bogey but McIlroy couldn't convert a four-footer to take the hole.
They remained all tied after birdies on the second and third holes before both players made a mess of the fourth.
McIlroy blasted his drive into an unplayable lie in the desert and ended up with a bogey.
But despite being in the fairway with wedge in hand Mahan dumped his approach into a bunker and then couldn't get up and down.
After another halve on the fifth Mahan finally opened up an advantage on the par three sixth when he carved his tee shot to gimme range and McIlroy missed the green and failed to chip in.
Mahan pushed the lead to two on the next hole when he was just a little less terrible than his opponent.
The American's bogey was good enough for the win after McIlroy missed the green on approach, chipped short and had the ball run back to him with his third and then failed to make a 10-footer after finally finding the surface.
After the eighth McIlroy found himself in a deep hole when a Mahan three-putt was enough to make it three winning holes in a row.
It was even deeper when Mahan's approach shot on the 10th hole almost found the cup and the 22-year-old McIlroy couldn't sink his 11-footer, leaving the American with a formidable 4-up lead.
McIlroy pulled holes back on the 11th and 14th but couldn't find an extra gear leaving Mahan to close it out on the 17th.
In the battle for third place American Mark Wilson prevailed over Westwood 1-up.
- AAP
