Tomasulo finished the tournament at 9-under-par 275 for a two-stroke victory over Aaron Barber, who was even-par in his final round at Redwood Meadows Golf and Country Club.
Nathan Green (70) and David McKenzie (73) shared third place at minus-6, while Steven Gangluff (70) and Cameron Percy (70) finished one stroke further back in a tie for fifth.
Making just his third Nationwide Tour start, the 23-year-old Tomasulo shattered the course record by four strokes with a 10-under-par 61 in the second round on Friday.
'Shooting the 61 gave me a lot of confidence that I could get the job done,' said Tomasulo, who shot a 72 on Saturday and shared third place at minus-7 entering the final round.
'You always hope it is going to happen this fast, but to actually get it done feels so good,' added Tomasulo, who collected $81,000 for the victory. 'It is great to get out here and get a win so quickly.'
Tomasulo moved to 9 under with birdies on his first two holes, but then dropped three strokes with a double bogey at the sixth and a bogey at the seventh.
He rebounded with a birdie at No. 8, but gave that stroke right back with another bogey at the ninth, making the turn at minus-6.
Tomasulo then played the back nine at 3 under to reach his championship score, collecting birdies at Nos. 11, 16 and 18. He drained a 6-foot birdie putt at the last to clinch the two-stroke victory.
'It was fun and tough all at the same time,' Tomasulo said of his up-and-down round, which gave him the highest winning score on tour this season. 'It was stressful out there. The course was playing so tough.'
Tomasulo, a Canadian Tour regular who has five straight top-five finishes on that circuit, earned full-time status on the Nationwide Tour for the rest of this year and 2006.
'All of the perks that come with winning is all a little overwhelming right now,' he said. 'It hasn't all sunk in yet.'
The 32-year-old Barber, who played on the PGA Tour from 2003-2004 and competed in this year's U.S. Open, had missed the cut in six of his 16 Nationwide Tour starts this season.
But Barber was just good enough Sunday to move up from a tie for third place overnight into the runner-up slot. He birdied his first two holes, dropped three shots with three bogeys between holes 12 and 16, and then reached even-par for the round with a birdie at the 17th.
'Finishing second place goes a long way on the money list,' said Barber, whose previous best finish on tour was a tie for third at the Rheem Classic in May. 'Peter played so well, and there wasn't much we could do to catch him there at the end.'
Keith Nolan (71), Alan McLean (71) and Jason Schultz (72) shared seventh place at minus-4. Michael Harris (67) and Tom Carter (69) tied for 10th place one stroke further back.
Overnight leader David Branshaw stumbled to a 9-over 80 in his fourth round to finish in a tie for 18th place at even-par 284.