Potter put a little more loft on his shot and holed it for a birdie and a two-shot lead.
He wound up winning by three shots over Johnson (72), Mickelson (67), Day (70) and Chez Reavie (68). That changed when he tapped in for par on the 18th hole, and he choked back emotion of a week he won't soon forget.
Potter stepped awkwardly off a curb at the Canadian Open in 2014 and broke his ankle so badly that he didn't play another tournament until Canada two years later. Yes, he was the co-leader after 54 holes and yes, he had a TOUR win under his belt from the 2012 Greenbrier Classic but the Floridian was still amongst exalted company.
Brantford's David Hearn is still looking to make his first cut in 2018 on the PGA Tour. Johnson, Day and Mickelson were among those who tied for second.
"It's been a struggle", he said. It's unbelievable right now. "This has been a blast this week".
Potter, who came into the week at No. 246 in the world, earns a trip back to the Masters for the first time in five years. He finished at 17-under 270 and earned $1,332,000, almost half of his career earnings going into the week.
Having looked exhausted after a prolonged day where every match he played went the distance or to extra holes but he was magnificent in the final, with the highlight being this eagle two at the fourth hole.
Potter overcame a three-putt bogey on the opening hole by not dropping a shot the rest of the way.More news: Boy in ambulance hit by driver now fighting for life
The left-hander picked up 11 strokes through 15 holes, only to bogey his final two holes for a nine-under 62. Needing an eagle to have any chance, his approach to the 18th was a few yards short and found a bunker.
Johnson was never really in it after his bogey on No. 8 to fall three shots behind. "I get my second career win after five years away from it".
"I didn't feel like I hit bad shots coming in there on the last couple, making bogeys there, but I just knew it was going to be tough coming in", Potter said.
An opening 77 at the more hard of the three courses used for the tournament, Spyglass Hill, did the damage and while steady efforts of 70-70 at Monterey Peninsula Country Club and Pebble Beach were credible it was too little too late.
Mickelson began his move late with three birdies in a four-hole stretch ended at the par-3 17th.
Having only chose to play last Sunday, [26.0] chance, Kiradech Aphibarnrat has won the second renewal of the World Super 6 Perth on the European Tour. I did at least manage to lay a nice chunk back on Reavie at [3.6] so it was far from a disastrous week.
Defending champion Jordan Spieth got a bobblehead doll in his return to Pebble Beach, but failed to capture the crystal trophy and finished nine strokes off the pace.
The other tournament also was a runaway.