Jaime Gomez, a winner earlier this year at the Corona Mazatlan Classic, fired a 4-under 67 on Sunday to take second place at 7-under-par 277. Daniel DeLeon was one stroke further back after a closing 73.
Anders Hultman took fourth place at 4-under-par 280 after a 5-under 66. Marcus Jones completed the event at minus-2, while Derek Gillespie was the final player in red figures at 1-under-par 283 after a 5-under 66 on Sunday.
Williamson got off to a good start with a birdie on the second. However, he faltered to bogeys on four and five to slide back to minus-8.
The 26-year-old slid even further after a bogey on the 10th. Williamson righted the ship with a birdie on 15. He got back to minus-9 with a birdie on the 17th.
Williamson drove behind a tree at the 18th. However, he managed to salvage a bogey to earn a one-shot win at Pine Ridge Golf and Country Club.
'Jaime didn't care what we were doing today, he was going to have a great round with or without us,' said Williamson. 'I heard footsteps all day. Everyone thought it was just a two-man race between me and DeLeon, but Jaime didn't quite see it that way.'
Gomez came flying up the leaderboard early in the round with birdies at two, four and five. He stumbled to a double bogey at the eighth, though.
The 38-year-old caught fire on the back side. He birdied the 12th and 13th to get to minus-6. Gomez kept flying, as he also birdied 14 and 15 to move to minus-8. However, he missed a 4-foot par putt at the last to end one stroke back at minus-7.
'Man, we fought all day long,' said Gomez. 'I knew I had a good back nine in me, and I played my best round of the week back there. We came down the stretch and had a chance. I hit the putt (on 18) right where I wanted to, I guess I just misread it. But you have to hand it to Lee. He played well all week.'
Bobby Kalinowski, David Mathis and Rob McMillan each closed with rounds of even-par 71. They finished at even-par 284 alongside Brien Davis, Ryan Ellis and Mike Grob.