And he's never had a better opportunity than this one.
Despite a bogey on the final hole of a taxing Saturday afternoon, Mickelson was the sole survivor to par at Merion with an even-par 70 that gave him a one-shot lead over Hunter Mahan, Charl Schwartzel and Steve Stricker going into the last round.
It's the first time Mickelson has held the outright lead through 54 holes in the U.S. Open, and the timing could be right.
Mickelson celebrates his 43rd birthday Sunday - on Father's Day, no less. He left Merion on Monday and didn't return until three hours before his tee time on Thursday so he could attend the eighth-grade graduation of his oldest daughter.
"It's got the makings to be something special," Mickelson said. "But I still have to go out and perform, and play some of my best golf."
Mickelson, who already has a record five silver medals for being runner-up at this demanding major, was at 1-under 209.
And the fun is just getting started.
"It's a hard challenge, but it's a lot of fun," Mickelson said. "Every shot requires such great focus because a penalty can bite you quickly. I can't wait to get back and playing. I feel good ball-striking, I feel good on the greens. I think it's going to take an under-par round tomorrow."
Saturday was more about weeding out the pretenders for this U.S. Open - and one of them turned out to be Tiger Woods. He started out just four shots out of the lead, and made a bending, 12-foot birdie putt on the opening hole. It never got any better for the world's No. 1 player. He made seven bogeys the rest of the way and didn't add another birdie, matching his worst U.S. Open score as a pro with a 6-over 76.
Woods was 10 shots behind.
"It certainly is frustrating," said Woods, who has been stuck on 14 majors since winning the 2008 U.S. Open at Torrey Pines. "I'm playing well enough to do it, and unfortunately just haven't gotten it done." The final hour might have been a sneak preview for Sunday. At one point, there were five players under par, and suddenly there was only Mickelson.
Luke Donald had the outright lead until two bad swings on the last two holes - a 2-iron into the bunker on the 17th that led to bogey, and another 2-iron into ankle-deep rough well right of the 18th green that led to a double bogey. Just like that, one of the best rounds of the day turned into a 71, and he was two shots behind. "I should have done better," Donald said. "It was disappointing, but I'll take the positives out of today - a really solid 16 holes of golf, and I'm only two back."
Hunter Mahan let his spectacular back nine filled with four birdies go to waste with a bogey-bogey finish for a 69. He will be in the final group for the first time in a major with Mickelson, whom he considers a close friend.
Former Masters champion Schwartzel also went bogey-bogey at the end of his round for a 69. Stricker made a 10-foot par putt on the 18th hole to complete a 70 and perhaps the steadiest round of the day. His only mistake in a round that lasted 5½ hours under sunshine was a tee shot into the water on the par-3 ninth for a double bogey.
At 46, Stricker can become the oldest U.S. Open champion.
"I've got to play smart golf ... not make any mistakes," he said. "I think that's the biggest thing. And it's a course where it's tough to come back."
Billy Horschel, tied with Mickelson at the start of the third round, kept his emotions in check and shot 72. He was two shots behind, along with Donald and Justin Rose, who also had a bogey-bogey finish. Rose thought his shot into the 17th was pure until it ran through the green into a sticky lie in the rough.
The third round featured so much movement, and so many wild swings, that seven players had a share of the lead at some point. Even though USGA executive director Mike Davis said the course was set up to allow for good scores, this was more about hanging on for dear life.
There was no faking it Saturday afternoon.
Thirty players were separated by only five shots at the start of the third round. By the end of the day, there were just 10 players separated by five shots, including amateur Michael Kim. He was tied for third until losing four shots on the last three holes.
That's really what Merion demands - score early and try not to lose too many shots at the finish. For all the talk about Merion being just a short course, the final two holes were beastly - 253 yards for a par 3 surrounded by deep bunkers and framed by the Scottish broom grass, and then a 530-yard closing hole up the hill, deep rough on both sides with bogeys or worse waiting for a single missed shot.
Stricker, remarkably, played bogey-free on the back nine. Horschel, striving for perfection at a championship that doesn't allow for it, dropped only one shot.
"Seventeen and 18, you've got to buckle up and hit good shots," Horschel said. "So I think tomorrow, with the pressure being on, those holes will stick out even more."
The 17th was crucial for Mickelson, who stood on the tee box one shot behind. He selected a 4-iron and couldn't have hit it any better.
"I just stood and admired it," Mickelson said. "It was one of the best shots I've ever hit. I mean, it just was right down the center of the green and I was hoping it would kind of get the right bounces. It left me a beautiful uphill putt that I could be aggressive with and I made it. That was fun to do that because that's just not a hole you expect to get one back."
Mickelson chose not to carry a driver, and he had to be flawless again on the long closing hole. He swung the 3-wood with confidence throughout the back nine and drilled another. With some 250 yards left, another fairway metal took him just over the green. His chip came out some 10 feet short and he missed the par putt to end a streak of 12 holes without a bogey.
But he still had the lead. It was the first time only one player remained under par through 54 holes at the U.S. Open since 2007 at Oakmont, when there was none. Mickelson was tied for the 54-hole lead at Winged Foot in 2006, where he lost a one-shot lead on the final hole by making double bogey.
Of his five runner-up finishes, that one stung the most.
But he's back for another try to win his national championship. Of those 10 players within five shots of the lead, Schwartzel is the only one with experience in winning a major championship. The challenge, however, hasn't changed from the opening tee shot on Thursday.
It's not any player. It's Merion.
Mickelson has one piece of history working against him. In the four previous U.S. Opens at this classic course, no one with the lead going into the final round has ever gone on to win.
"I love being in the thick of it," Mickelson said. "I've had opportunities in years past, and it has been so fun, even though it's been heart-breaking to come so close a number of times and let it slide. But I feel better equipped than I have ever felt heading into the final round of a U.S. Open."