On that sunlit Sunday afternoon, Mickelson played a game that was beyond compare. Coming down the stretch, he was four under par over the final six holes, a staggering achievement on what is generally regarded as the most brutal finish on The Open rota. Having started five shots back, he signed for a 66, the lowest round of the championship, and won by three strokes.
Winning at Muirfield left Mickelson needing just a US Open to complete a career Grand Slam of Major titles. It also demonstrated that a player once renowned (and often derided) for aggression and recklessness had finally acquired the patience and maturity demanded by a links course of the quality of Muirfield. That evening, as he stood on the 18th green with one arm cradling the Claret Jug and the other round the shoulder of his wife Amy, Mickelson had the golfing world at his feet.
Never the best place for it. Golf has a particularly nasty way of biting back at those who seem to have mastered its demands, and Mickelson felt its teeth in the weeks and months ahead.Victory at Muirfield last year moved him into second place in the world rankings - astonishingly, he has never been No 1 - but he has spent the 50 weeks since that moment tumbling down the global pecking order. He arrives in Scotland for this week's Aberdeen Asset Management Scottish Open at Royal Aberdeen holding 13th place, the consequence of his failure to finish in the top 10 at any tournament since he was third at the Abu Dhabi HSBC Championship in January.
However, he also comes as reigning Scottish champion, a status that will draw far more attention than his recent form merits. Mickelson has never exactly minded the limelight, but he could surely do without it at a time when he ought to be sharpening up his game for the following week's Open title defence at Hoylake. Mickelson's triumph at Muirfield was based squarely on the groundwork he had put in at Castle Stuart, last year's Scottish Open venue, and a little more graft at Aberdeen may yet pay dividends.
Yet Mickelson's biggest problem this year has been on the greens. A streaky putter throughout his career, a weakness disguised by the wizardry of the rest of his short game, the best that could be said of the 44-year-old is that he has finally acquired some consistency. This year his putting has been unrelentingly poor.
It reached a kind of nadir at last month's US Open at Pinehurst. As always, Mickelson went into the tournament with a huge wave of emotional support behind him; and, as always, he came up short. That in itself was no great surprise - he has finished second in his national championship a record six times - but on this occasion it was not his old wildness or a crazy attempt at a miracle shot that did for him. Rather, it was in the simple business of poking the ball into the hole that he fell down.
It hardly helped that he was in the midst of a swirl of rumours and allegations about insider trading at the time (he has since been pretty much cleared) but off-course issues could hardly be considered critical when the rest of his game was in fine shape. From tee to green, Mickelson was every inch a contender, but over the last few feet he was an also-ran. He missed a succession of five-footers over the course of the tournament, and dynamited any prospect of a challenge for the title with three three-putts during his Friday round of 73.
"My putting was pathetic," Mickelson said after the Fedex St Jude Classic, the tournament that preceded his trip to Pinehurst. "You can't win any golf tournament putting the way I've been putting."
His first answer was to turn to the claw grip in pursuit of a smoother action. Next, he made an adjustment to his eye alignment. Neither change had the desired effect. By the end of the US Open he had reverted to his old way of doing things.
"It's a frustrating thing for me," Mickelson sighed. "It's a very important part of the game. Whenever you putt well and you make the short ones and you make those five and six-footers and you're running a couple of 20-footers in, the game feels easy. You don't put pressure on yourself to hit it close.
"You can hit more of the middle of the greens then. Your ball striking then becomes a lot easier because your targets are a lot bigger. But the hole looks like a thimble to me right now. I'm having a hard time finding it."
Yet maybe a trip to the east of Scotland will unlock something deep within that intriguing psyche of his. A good player who is putting badly knows what he has to do to get his game in order, and Mickelson's touch may return just as suddenly as it left him.
Royal Aberdeen is the sort of classic course where such a thing might happen.
For those who admire Mickelson's wondrous gifts it has always seemed absurd that he ties himself up with the mechanics and technicalities of his sport, but the Balgownie links seems likely to appeal to his more instinctive side.
"I was just trying to see the shot and create it," said Mickelson recently of last year's final round at Muirfield. "You have so much running through your head that you want to try and simplify it as much as you can."
Maybe it's time for him to follow his own advice.
o Scottish Open, Thursday until Sunday 13, Royal Aberdeen Golf Club. Tickets from £11 to £31.