The win came a year after Mahan finished second in the same tournament, and three days after he shot a 62 on the same course to take a first-round lead.
"Knowing that you can win and actually winning are two different things,'' he said. "And to win the way I did is just amazing to me. To have to birdie in a playoff, especially after he hit that shot in there is mind-boggling.''
Mahan 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.''
Mahan earns just over $1 million for the win and a spot in next year's Mercedes-Benz Championship in Hawaii and the Masters.
Williamson gets $648,000 for second. Coming into the weekend, Williamson was seventh on the Nationwide Tour money list with $153,249 in earnings. His play here also earns him a trip to Flint, Mich., for this week's Buick Open. He was hoping to earn back his PGA Tour card with a win, but 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.''
He said he's hoping other PGA events will now give him exemptions.
"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.
This was the seventh playoff on the PGA tour this season, and the 19th in the history of this tournament.
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.
It was Singh's third best finish of the season, behind his two victories. He said he plans to take next week off and get an MRI on a sore right elbow.
"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-three eighth hole.