Seamus Power's challenge at the The American Express fell away after a difficult start to his final round, with Hudson Swafford prevailing after the American carded an eventful 64 at La Quinta Country Club.
Power began the day just two off the lead but got off to a disastrous start with a double bogey and a bogey on his opening two holes to fall out of contention.
He lost 3.2 strokes Sunday from tee-to-green and carded a closing 74 to slip into an eight-way tie for 14th in a group which included world number one Jon Rahm. Graeme McDowell finished on two under par in a tie for 40th.
Swafford began the day three shots off the lead and fell further behind with a bogey on the first, but bounced back to birdie the second, fifth, sixth and eighth to remain in contention.
The 34-year-old then started the back nine with a hat-trick of birdies and followed a bogey on the 13th with another birdie on the next to lead by two, only to drop another shot on the 15th.
With Brian Harman making a birdie on the 18th and former Open champion Francesco Molinari doing likewise on the 16th there was suddenly a three-way tie for the lead, only for Swafford to hit a superb approach to the par-five 16th to set up an eagle from eight feet.
A birdie on the 17th effectively sealed the win and Swafford made his only par of the back nine on the 18th to finish 23 under par, two shots ahead of compatriot Tom Hoge.
Harman shared third place with Lanto Griffin and Lee Hodges, while Molinari's bogey on the last meant the Italian had to settle for a closing 68 and a tie for sixth.