By Mark Lamport-Stokes
While world number one Tiger Woods and second-ranked Phil Mickelson returned scores they would prefer to forget, Mahan fired a six-under-par 64 to seal his first victory in an elite World Golf Championships event and his third on the PGA Tour.
"I feel great," Mahan said greenside after booking his place on the U.S. team for the October 1-3 Ryder Cup. "This feels unbelievable."
Three strokes off the pace going into the final round, the 28-year-old Mahan charged into contention with a sizzling front nine of five-under 30 and made two clutch putts to save par over the last four holes to post a 12-under total of 268.
Sean O'Hair, who was level with Palmer at the start of the day, returned a 71 for fifth place at eight under.
Palmer and O'Hair each led outright early on in the final round but Mahan made the most significant move with his red-hot run to the turn to draw level with O'Hair at 11 under.
O'Hair slipped back with a three-putt bogey at the eighth and Mahan rolled in an 11-foot birdie putt at the 13th to establish a two-stroke advantage which he never relinquished.
He coolly sank par putts from 15 and eight feet at the 15th and 17th, although in between he was very fortunate not to drop at least a stroke at the par-five 16th.
Mahan was given a free drop there after his second shot ended up in a flower bed to the right of the green. He chipped 60 feet beyond the hole before two-putting to save par.
"I was nervous," he said of the closing stretch. "I wanted it (victory) so bad and I was playing so good. I just didn't want to make any bogeys. I didn't want to give them (his rivals) any momentum out there."
Woods, a seven-time winner here, produced his worst finish on the U.S. circuit after closing with a 77 while Mickelson squandered a golden opportunity to replace his fellow American as world number one.
Left-hander Mickelson would have reached the top of the rankings for the first time by finishing alone in fourth place or better.
Having begun the final round tied for 10th, he tumbled into a share of 46th at three over after battling to a 78 that featured a double-bogey at the ninth where he three-putted from four feet.
"It's hard to be upbeat after a round like today," said Mickelson. "I've got some work to do to get my own game sharp."
Woods, who ended his round almost three hours before the overnight leaders teed off, produced his highest final score as a professional to share 78th place in the 80-player field.
At one of his favorite venues, he carded two double-bogeys in the last five holes to post an 18-over total of 298, 30 strokes behind Mahan.
It was his highest 72-hole aggregate on the PGA Tour, his worst relative to par and the first time since the 2003 PGA Championship he had strung together four rounds over par.
"(I'm) just not playing well," a grim-faced Woods said after totaling 33 putts. "The only thing I can say all week is I was patient, and unfortunately that's not enough."
(Editing by Frank Pingue)