Mickelson, who won his fourth major title, ended the day at 16-under 272, three shots ahead of third-round leader Lee Westwood, who shot a 71.
Mickelson posted the win thanks to some amazing shots on the Augusta National Golf Club back nine. Much as is his routine, his drives were sprayed around and he was forced to make several improbable recoveries -- two of them coming on the back-nine par-5 holes.
"This has been a special day," Mickelson said after the win. "It's something I'll always cherish."
Mickelson's third championship tied him with Sam Snead, Gary Player, Nick Faldo and Jimmy Demaret. He trails only Jack Nicklaus, who has six Masters' titles, and Arnold Palmer and Tiger Woods, with four each.
Anthony Kim finished third after firing a 7-under 65 for 12-under 276 while in his return to competitive golf Woods shot a 3-under 69 Sunday. He finished tied for fourth with K.J. Choi, who also had a 69, at minus-11.
Woods expressed disappointment at his rustiness after a four-month absence due to his marital infidelity scandal.
"I entered this event and I only enter events to win and I didn't get it done," Woods said. "I didn't hit the ball good enough and I made too many mistakes around the greens, consequently I'm not there."
Woods, however, did manage to sink two eagles Sunday, one on the seventh hole and the other on the par-5 15th. The holes allowed him to tie a Masters record with four eagles in a tournament.
Fred Couples, 50, finished with a 2-under 70, coming in sixth at 9-under 279.