Instead, Lefty settled for a familiar spot.
Mickelson closed with a 3-under 69 that left him tied with Justin Rose, four shots behind the wire-to-wire winner.
"It was just a good, solid round of golf," Mickelson said. "I needed something exceptional."
The 21-year-old Spieth won with an 18-under 270, tying the Masters record for lowest overall score.
Mickelson and Rose finished at 274, a score that would have been good enough to win the last three years. In fact, it was lower that Mickelson's score in two of his three Masters victories.
"The fact is, I would have taken 14 under at the start of the week," he said. "I played really well to shoot 14 under and I simply got outplayed by a young player who just played some incredible golf."
On a resume highlighted by five major titles, it was Mickelson's 10th second-place finish in golf's biggest events.
It also completed a Grand Slam of sorts, one he would prefer not to have.
Mickelson has now finished second in every major championship.
This won't hurt nearly as bad as some of the others, especially all those close calls in the only major Mickelson has never won, the U.S. Open.
At 44, Mickelson hasn't played all that well in recent years on the PGA Tour. But he knows how to get up for the biggest events, having won the British Open in 2013 with a stirring final-round comeback, and finishing second in the last two majors going back to the 2014 PGA Championship at Valhalla, where he was one stroke behind Rory McIlroy.
"I don't have a great explanation other than I really focus on those events," Mickelson said. "It's not my motivation to go out and try to grind out wins week after week. I want to zero in on our four or five biggest events, and I've been fortunate in that I've been able to get some of my best golf out of those events."
Spieth dominated this Masters from start to finish. Mickelson started the final round five shots back and never got within four shots of the lead, even after holing out an eagle from the bunker at the par-5 15th hole.
He was doomed by three bogeys spread throughout the round, keeping the popular player from really getting the Augusta patrons on his side. They cheered him all the way, of course, but never unleashed one of those really big roars that would have signaled Lefty, playing just ahead of Spieth in the next-to-last group, was making a serious move.
"Every time I got a birdie here or there, I stalled with a bogey," Mickelson said. "It was a really fun tournament. I thought I played some good golf. I just got outplayed. Jordan was phenomenal."
Rose also played extremely well, and he was the only one who got as close as three strokes to Spieth in the final round.
Carrying on the momentum he had Saturday, when he closed with five birdies on his last six holes to get into the final group of a major for the first time, Rose birdied the first two holes Sunday in what looked for a while like essentially a match-play scenario with Spieth.
But Rose stalled, playing the next 10 holes at 2 over. He gave himself a glimmer of hope with three straight birdies starting at No. 13, and a last gasp came at the par-3 16th, when he stuck his tee shot to 15 feet for another birdie try, while Spieth faced a dicey 8-footer to save par.
Rose missed his putt. Spieth made his.
"It was probably one of the best putts he hit all day," Rose marveled. "I was looking for that two-shot swing to keep it interesting."
Rose, the 2013 U.S. Open champion, posted his highest finish ever at Augusta National, where he's made the cut in all 10 of his appearances and finished in the top 15 four other times.
Asked what it's going to take to finally break through, he smiled.
"Keep shooting 14 under," Rose said.
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