Mickelson approached the 441-yard 18th hole with a shot at a major tournament record 62.
"I want to cry", Mickelson said. His putt on the last for an all-time low kissed the cup, birled round it a bit and stayed out.
The 46-year-old American knows these sorts of chances don't come along very often. At his age, especially, the odds are against Mickelson ever coming that close again. He was 3 over par. This was the 28th 63 in major championship history but nobody has dipped below it.
And he had reason to be disappointed. Mickelson plopped his hand on his forehead in disbelief. He broke the course record. And he likely will have the lead. With golfers from the United States winning the last six Opens staged here at Royal Troon, the sight of eight Americans in the top 11 already is an ominous sign.
One name stands out, however.
Spieth faced two 10-foot birdie putts on his opening holes.
The four-time major victor was pleased to be able to focus on his bid for the Claret Jug after a build-up to the tournament dominated by his and numerous other stars' decision to withdraw from the Olympics. We've had a lot of moments that we've shared together and that was another one that was really fun.
Steve Stricker was nervous when he teed off at Royal Troon, and it showed. He had to scramble for par on the first hole and took bogey on the next hole.
The British are represented by Andy Sullivan at four under and Justin Rose three under.
And it might be his last major.
Rory McIlroy shot 69, Jordan Spieth and Dustin Johnson shot 71 and Jason Day had 73.
He might not see it for long here either judging by the forecast but in this shimmering setting, the first round of the 145th Open Championship unravelled. Except this was the British Open, and the conditions were never easier at Royal Troon.
It was still good enough for an eight-under effort and a bogey-free round and put him three ahead of compatriot Patrick Reed and Martin Kaymer of Germany, with a host of players one stroke further adrift on four-under. The high points of his round came early, when he birdied three and six, but they lost their lustre as he accumulated bogeys on five, eight, nine and 18. 10 and 11. He is level par through 13 holes. Rory McIlroy was the best of them at three under. He missed last year's tournament because of injury. Greg Norman shot 69 in the second round of the 1996 Masters, and Raymond Floyd also shot 69 in the second round of the 1982 PGA Championship at Southern Hills. "I thought I hit some good shots on the back nine, but it was nice to get a birdie and get it back into the 60s". A little-known 23-year-old, he was the surprise player of the day. It would have been easy to score 78 there, simple. Birdies had been flying in wild abundance on the front nine. As many as 74 players shot par or under.
Thomas was the first to post a low score after he birdied his first four holes. "It's going to be tough if it's not good". Justin Thomas and Patrick Reed are leading the way. Alas, Monty couldn't keep it going. "I love it. That's why I was here last week (Scottish Open) playing at Castle Stuart in that stuff".
Luke Donald, playing alongside Montgomerie in the first group, was at 2 under. He didn't miss too many, and just managed himself so well.
Colin Montgomerie started with a buried lie in a pot bunker on No. 1 and made double bogey. He was third at the 2004 Open at Troon.
On a sunny day, McIlroy finishes two under in the opening round of British Open.
Former Auburn player Blayne Barber was a stroke back at 66 along with Martin Laird, Ben Crane, Shawn Stefani, Matt Bettencourt, Michael Kim and Lucas Lee.
There wasn't an empty seat in the grandstand next to the first tee, where the silver claret jug was on a podium.