His hand went to his head in disbelief. Groans could be heard as far as Glasgow. No one was as distraught as the man in black.
"It was one of the best rounds I've ever played," Mickelson said after taking a three-shot lead in the first round of the 145th Open Championship.
"I was able to take advantage of these conditions, and yet I want to shed a tear right now.
"That putt on 18 was an opportunity to do something historical. I knew it, and with a foot to go I thought I had done it. I saw that ball rolling right in the center.
"I went to go get it, I had that surge of adrenaline that I had just shot 62, and then I had the heartbreak that I didn't and watched that ball lip out. It was, wow, that stings. It was just heartbreaking."
To say the American made hay while the sun shone is an understatement. The 46-year-old was haymaker-in-chief in Scotland on Thursday, and helped himself to a scintillating 63 to lead from compatriot Patrick Reed and Germany's former world No. 1 Martin Kaymer at the sole major played in Britain.
The big yellow orb that bathed Royal Troon in a warm glow has been an infrequent visitor this UK summer and is not set to stick around.
"I haven't seen a day like this in some time. I've been here 11 days, the wind is non-existent and the sun is out. That was weird," Mickelson said.
England's world No. 11 Justin Rose, tied for 12th after an opening 68, added: "I thought all the lights were on in the house when I woke up. It looked different. I realized it's actually the sunshine."
With only a gentle zephyr blowing across the Firth of Clyde from beyond the Isle of Arran in the west, the Troon candy jar was left open and Mickelson helped himself. His perma-grin suggested he couldn't quite believe it either.
The left-hander, with the slouch of a gunslinger and the gait of a farmer, has not won a tournament since he clinched the Claret Jug at Muirfield in 2013 for his fifth major.
But his opening round had that magical quality reminiscent of the final day over on Scotland's east coast when he proved -- to himself as much as anybody else -- that his game could translate to the demands of links golf.
He roared to the turn in 32, including a birdie at the tricky par-three eighth -- known as the Postage Stamp -- which cost Bubba Watson a triple-bogey six. With the crowd firmly behind him, Mickelson had made four more birdies by the time he got to the 18th.
Only a handful of players all day had bettered their outward half, given Troon's back nine plays into the prevailing wind. Walking up the final fairway, Mickelson said to caddy Jim "Bones" McKay that he needed his "best read" on the final 18-foot putt.
"I don't know if you know this..." he said to his longtime bagman. "Oh, I know," replied Bones, who toppled backwards in despair as the putt failed to drop.
Mickelson's miss put him alongside 27 other players to have shot 63 in a major. He joked that there must be a hoodoo on anyone firing 62.
"There's a curse because that ball should have been in," he said.
Asked if he believes in the golf gods, he replied: "I didn't, but I do now."
With the build-up to the tournament dominated by talk of the latest withdrawals from the Olympics, Mickelson's marauding has put the emphasis firmly back on golf.
He was six shots clear of Rory McIlroy, eight in front of world No. 3 Jordan Spieth and U.S. Open champion Dustin Johnson, and 10 ahead of top-ranked Jason Day, but there is much work to do in the next three rounds, with poor weather forecast for Friday at least.
"One of the biggest challenges when you shoot a round like this, you start expectations running through your head, and that's the one thing that I'll have to try to suppress and hold off," Mickelson said.
"It's going to be very difficult. Changing conditions, we'll have different winds. Hopefully I've prepared myself well enough to tackle this golf course under those conditions."
Only Old Tom Morris -- 46 years and 99 days when he won just down the road at Prestwick in 1867 -- can claim to be an older Open champion than Mickelson would be were he to win on Sunday.
The rest of the field will be hoping the haymaker gets floored before then. Or that the curse returns.