DiMarco shot 2-under-par 69 to finish at 17-under 267, one shot clear of Kenny Perry (70) and Kaname Yokoo (64).
John Daly (70) tied Lee Janzen (64) for fourth place at 15-under. Daly earned a share of the early lead, thanks to an eagle and a birdie over his first three holes. He also recorded an eagle and a birdie over his final four holes; however, he played Nos. 5-11 in 5-over.
Overnight leader Duffy Waldorf struggled to a 2-over 73 and finished in sixth place at minus 14.
DiMarco earned $720,000 for his third career PGA Tour title, putting him atop the 2002 money list at almost $1 million. His total stands at $986,857 after four events.
DiMarco has three top-six finishes in four starts this season. He was one stroke off the 54-hole lead at the Mercedes Championships, only to bogey both par-5s down the stretch Sunday to tie for fifth.
He then finished tied for sixth at last weeks Bob Hope Chrysler Classic after hitting into the water on both the 17th and 18th holes in the final round.
It appeared another final-round failure was in the cards this Sunday.
DiMarco birdied four of his first eight holes to take a four-shot lead at 19-under. But he left his game at the turn.
He pushed his tee shot into the right rough at the par-4 11th. With a tree impeding his swing, he was able only to advance his second shot some 15 yards ' still in the rough.
He made double bogey, cutting his lead in half.
Trying to avoid the water on the right of the par-3 12th green, DiMarco next pulled his tee shot into the left-hand greenside bunker. He failed to get up and down for par. Meanwhile, Perry sank a 20-foot birdie putt ' his first of the day ' to tie for the lead at 16-under.
DiMarco again found trouble off the tee at the par-5 13th, pushing his drive into the right water hazard. He went on to make bogey, but got a bit of a reprieve when Perry missed a two-foot birdie putt.
'That really killed me,' Perry said. 'I missed a gimme.'
Leading outright for the first time, Perry was quickly caught by Yokoo, who shot a back-nine 31 to enter the clubhouse at 16-under.
DiMarco reached that number with a birdie at No. 14, yet couldnt match Perrys birdie at the par-5 15th. Perry hit the green in two shots and easily two-putted for a birdie 4. DiMarco, on the other hand, had to settle for a par, after his tee shot caromed off the cart path and settled behind a bush.
DiMarco showed a great deal of resolve on the par-3 16th. He stuck an 8-iron from 162 yards to within three feet of the hole. And just before he attempted to tie for the lead, someone in the crowd yelled, Noonan! ' a choking reference from the movie 'Caddyshack.' DiMarco stayed over his putt and rolled in the birdie effort. He then motioned for security to throw out the boisterous patron.
The final group was forced to endure a 15-minute wait on the par-4 17th, as players in front of them were taking their shots at the 332-yard hole.
During the downtime, DiMarco, a Pittsburgh Steelers fan, found out his team lost the AFC Championship game to the New England Patriots.
The same fate would not befall DiMarco.
Both he and Perry hit drivers off the tee. DiMarco came up short of the green, while Perrys ball rolled into a swell, right of the putting surface.
DiMarco pitched onto, and through the green with his second shot, but successfully saved par. Perry, though, came up 40 feet short of the hole on his second shot and three-putted for bogey.
'I don't know what you're supposed to do on that crazy hole,' Perry said. 'I was looking for the miracle shot, and it didn't happen.'
Leading by one stroke with one hole remaining, DiMarco two-putted from 45 feet for par, leaving Perry with a 30-foot birdie putt to force a playoff. He missed right.
Perry has now finished third at the Mercedes, tied for sixth at the Bob Hope and second this week. It was the third time this year he has played in the final group without winning.
Full-field scores from the Phoenix Open