PGA champion Yang, who started the day six strokes behind the lead, had also fired a 65 and was in control of the tournament before he threw away a two-stroke advantage on the 17th with a costly bogey when he pulled his drive into the water.
"This means a lot, to play the way I did down the stretch, (it) feels great," Mahan told reporters after coming from four back to win his first event since 2007. "I feel like great players win, and I want to be a great player."
The 21-year-old Fowler had missed a 30-foot attempt on 18 that would have given him a tie for the lead, though a decision to play cautiously on the par-five 15th when he chose to lay up with his second shot instead of attacking the pin was not a factor in whether he won or lost the tournament, he said.
"The way I look at it, it wasn't won (or lost) on 15. I thought I could hit a wedge up there (for a third shot) and give myself a chance," said Fowler.
"I would re-hit the wedge shot if I could."
Australia's Mathew Goggin, Chris Couch and Charles Howell III tied for fourth at 13-under.
Twice champion Phil Mickelson finished with a 67 to get to nine-under and a tie for 24th but is still looking for his first win of the year.
Overnight leader Brandt Snedeker bogeyed three of his first seven holes and was never a factor as he finished with a seven-over 78 and dropped into a tie for 43rd on seven-under 277.
(Reporting by Jahmal Corner in Los Angeles; Editing by Greg Stutchbury)