Mahan, in his fourth year on Tour, hit a 2-foot birdie putt on the first playoff hole to beat 40-year-old journeyman Jay Williamson.
Mahan, 25, had just one other top-10 finish this year, tying for fifth at the Shell Houston Open in April. But he plays this tournament well, posting one of his two career second-place finishes.
His 62 on Thursday was one shot off the course record, and his 71 on Friday was his first round out of the 60s in this tournament since he shot a 72 during the third round in 2005.
'After that 62, I felt it almost came out of nowhere. ... I felt like I could play good here, I know I can win.'
He shot a 65 Sunday. Trailing by a stroke on the final hole, he hit a 144-yard approach to within 7 feet. After Williamson missed a 12-footer for birdie, Mahan put his in the back of the cup, pumping his fist and sending the pair back to the tee.
Mahan then hit his 134-yard approach on the first playoff hole within 2 feet.
'I just kept plugging away,' he said. 'Jay gave me a little opening on 18 and I'm glad I made it.'
Williamson was just 7 feet away, but pushed it right.
'I hit it too hard and I hit it too high,' he said. 'I'm not proud of either putt, but I'm proud of the way I played today.'
Mahan earns just over $1 million for the win, and Williamson gets $648,000 for second. Coming into the weekend, Williamson was seventh on the Nationwide Tour money list with $153,249 in earnings.
The second-place finish will be enough to earn Williamson a trip to Flint, Mich., for this week's Buick Open. He was hoping to earn back his PGA TOUR card with a win.
'I really feel like if I was a great putter I would have won by a lot today,' Williamson said. 'But I know what I need to work on to get to that next level.'
Williamson has had 279 other PGA TOUR starts, and his best finish had been a tie for third in the 2003 BellSouth Classic.
A 1989 graduate of nearby Trinity college, where he played hockey and baseball, he said he learned a lot from this tournament.
'I learned that I can play with these guys,' Williamson said. 'And I learned that I'm going to be a golfer for a while.'
Nick O'Hern shot a 66 Sunday to finish at 11-under, good enough for third place and Vijay Singh shot a 65 for fourth.
Singh plans to take next week off, allowing him to rest a sore right elbow, and said he will get an MRI on Monday.
'I'll rest for a few days, and then back to business,' he said.
Mahan made four consecutive birdies, starting at No. 10 and led Williamson by 2-strokes coming down the stretch.
Williamson got back to within a stroke by making birdie on the 15th, and Mahan dropped into a tie for the lead with a bogey on par-3 16th.
Mahan then hit his second shot at 17 over the green, and two-putted for bogey.
Williamson went straight for the pin, and just missed a 20-foot birdie putt, burning the lip right. But his par gave him the lead going into the final hole.
Bo Van Pelt had the shot of the day, following up bogeys on the 14th and 15th holes by acing the 171-yard 16th hole with a 7-iron.
'Bill Haas had hit a really good shot with a 7-iron right before me, so I knew it was the right club,' Van Pelt said. 'It took one hop, and I think then it just kind of dribbled in -- pretty exciting.'
It was the second ace of this tournament and the 28th in the tournament's history. Craig Perks had a hole in one on Thursday on the par-3 eighth hole.
Van Pelt finished the tournament at 8-under, good enough to tie for sixth place.