On paper, Ted Potter Jr. did not stand much of a chance against Dustin Johnson heading into the final round of the AT&T Pebble Beach Pro-Am, tied at 14-under.
Potter, who won just over $207,000 (£150,000) a year ago, picked up more than $1m for his victory on Sunday. You don't know what is going to happen with your golf swing or your career. To come back from that it's just unbelievable right now.
Mickelson, who shot a five-under 67, Day (70), Johnson (72) and Chez Reavie (68) finished in a tie for second at 14-under 273. Potter chipped in for birdie on No. 7 and didn't drop a shot the rest of the way.
The victory was a remarkable comeback by Potter, who incurred a fractured right ankle in a "freak accident" in 2014 when he slipped on a path.
The left-hander played on the Web.com Tour past year to regain his PGA Tour card after time out with a broken ankle.
"He played great today, and this was special", said Streelman, who finished sixth among the pros and four shots back of victor Ted Potter Jr.
Potter Jr. was not considered a top-3 favorite entering the final round (though tied for the lead), and got off on the wrong foot with a bogey on the first hole.
Potter doubled his lead with a chip-in birdie from off the seventh green, as Reavie posted back-to-back gains from the eighth to reach the turn in 31 and join Johnson on 15 under.
Johnson was hoping for a victory this week to strengthen his grip on his world number one ranking.
Potter, who has spent time on the PGA and Web.com Tours in recent seasons, earned his second PGA Tour win - his first since the 2012 Greenbrier Classic.
"I'm going to keep the original, then I'm going to give him one made that he can put in his house next to all his other trophies".