Mahan took a two-shot lead with a birdie on the 13th hole and then protected it by saving par after hitting into a flower bed on the 667-yard 16th hole. It was one of five straight pars he made down the stretch.
Ryan Palmer ended up in second place after firing a 69, finishing two shots behind Mahan at 10-under 270. Retief Goosen (65) and Bo Van Pelt (67) finished in a third-place tie at 9-under 271.
Overshadowing the proceedings, however, were the struggles of Woods, who recorded his worst tournament as a professional. He closed with a 77, his worst final-round score since turning pro in 1996, finishing at 18-over 298 -- good for a 78th-place tie.
Woods, however, hung on to his No. 1 world ranking because second-ranked Phil Mickelson also struggled with his worst round of the season Sunday, laboring to an 8-over 78.
"It doesn't surprise me at all," Woods said of his performance. "It's been a long year."