Williamson closed with a 5-under 67 to finish at 16-under-par 272 for his second Canadian Tour win. His maiden victory came last year at the MTS Classic -- by one shot.
'This one was a little less nerve-wracking,' Williamson said, comparing his latest victory with his first. 'I knew if I could get to 17-under, it should be enough.'
Ben Hayes shot a 67 to move into a share of second place with Monday qualifier Justin Lee (69) at 11-under-par 277.
Stephen Gangluff, who shared the overnight lead with Williamson, struggled to a one-over 73. He finished tied for fourth with Brian Guetz, who closed with a 69.
The last three Canadian Tour events were all decided by a playoff, but Williamson made sure this one wouldn't be. He opened with a birdie at the first hole, then added birdies on three straight holes from the third to get to 15-under.
Williamson dropped a shot at the par-4 seventh, but got it back with a birdie at the 10th. He stumbled to another bogey at the 12th before collecting birdies on two of his final six holes, including one at the 18th.
'I never really let anyone get too close today,' Williamson said. 'There is trouble everywhere on this golf course, so I didn't want to let up.'
Williamson never really let up over the final two days of play. He entered the third round in 22nd place, but poured in eight birdies Saturday for a 64 that put him in a share of the lead.
His win Sunday was worth $16,000 -- good enough to put the American in third place on the money list behind Rob Oppenheim and Guetz.
'It's a comfort level for me,' Williamson said. 'I know I can win out here now.'
Oppenheim closed with a three-under 69 to finish tied for sixth place at 9-under-par 279. Joining him there were Dustin Risdon (66), Wes Heffernan (67) and Travis Johnson 71).