Vaughn Taylor fired a seven-under-par 65 in the final round of the AT&T Pebble Beach Pro-Am to claim his first PGA Tour win in almost 11 years.
Taylor took out back-to-back Reno-Tahoe Opens in 2004 and 2005 but had not been on the winning list since then.
The 39-year-old is 447th in the world golf rankings but managed to sink nine birdies in his last round for the unlikely victory.
With the win, Taylor earned his tour card -- something he has not had since 2012 -- for the next two years. He will also get to play at Augusta in April, which will be his first Masters trip since 2008.
"So many things are going through my head right now," Taylor told CBS. "I had so much fun out on the course this week.
"This is a life-changing victory, one that I didn't think was going to happen.
"I worked so hard and I'm at a loss for words right now. I didn't even have my tour card and now I'm going to be playing in the Masters. It's such a dream come true."
Taylor started the day six strokes behind Phil Mickelson, who shot four birdies and four bogeys to finish with an even-par 72.
Mickelson was aiming for a 43rd PGA Tour win - and his first since 2013 - but missed a birdie opportunity on the final hole to send the tournament into a play-off.
Jonas Blixt finished in third after his final-round 69, becoming the only person to finish with scores of under 70 in all four rounds.
Fellow Swede Freddie Jacobson finish tied for fourth along with Japan's Hiroshi Iwata, who carded a 72 in the final round.
Kang Sung was the biggest loser, shooting a 77 to finish equal 17th after he held the joint lead going into the third round following an impressive 60.