He finished at 16-under-par 264 and collected his first win on the Nationwide Tour. Williamson finished one clear of overnight leader Justin Bolli (70) and Garrett Willis (66).
'This means a ton,' acknowledged Williamson, who pocketed $94,500 for the win. 'Seeing your name on top of the leaderboard is always a nice thing. I just turned 40 and this really isn't a tour for 40 year olds. Obviously winning is a tremendous boost to my career.'
Jason Day fired an 8-under 62 in round four and tied for fourth place with Tee McCabe, who posted a 6-under 64 on Sunday. The pair came in at 14-under-par 266.
Williamson birdied every other hole starting at the first and ending at the fifth. He rattled off three consecutive birdies from the ninth and suddenly he was a factor in the tournament.
He birdied the 13th hole and was in the lead at minus-16. Williamson parred his final five holes, but had to wait because overnight leader Bolli had some chances coming home.
Bolli recorded two birdies and two bogeys through the first 16 holes. He birdied the par-3 17th and was tied with Williamson atop the leaderboard.
Unfortunately for Bolli, an errant approach into a bunker at 18 led to a bogey and gave Williamson his first win in a PGA TOUR-sanctioned tournament.
'I was trying to get aggressive from the fairway,' said Bolli. 'I wanted to stick it on the last hole and win the tournament. Today was win or bust, especially after starting with a three-shot lead. I have no regrets, it was just bad execution.'
'I hate to see a guy bogey the last hole for me to win,' said Williamson. 'But I feel like I deserved to win the tournament this week. I played well all week. I finally played well on Sunday, too.
'I've had three or four opportunities this year on Sunday to not necessarily win but make a good check and I haven't been able to capitalize. It feels good to finally get it done.'
Brad Elder (67) and Craig Lile (68) tied for sixth place at minus-13.
Patrick Sheehan (66), Adam Riddering (66), Andrew Johnson (68) and Michael Boyd (69) shared eighth place at 12-under-par 268.