Mickelson posted a 3-under 69 and won the title by a shot over Ernie Els. Mickelson finished at 9-under-par 279 and became the fourth player in tournament history to birdie the 72nd hole for the championship.
Mickelson broke an 0-46 drought in golf's four most important tournaments. He finished second twice at the U.S. Open and once at the PGA Championship. Mickelson took third the last three years at Augusta and finished in that position four times overall at the Masters.
That's all over now.
'In the past 10 years, to have come so close and fallen short or having people make critical putts against me, makes this difficult journey towards my first major title so much sweeter,' said Mickelson, who joined Mark O'Meara (1998), Sandy Lyle (1988) and Arnold Palmer (1960) as the only players to birdie 18 for the green jacket.
Mickelson matched Els at 8-under par with a 20-foot birdie putt at the par-3 16th, a hole that saw back-to-back aces earlier in the round. Els parred his final two holes, including a nice par save from a fairway bunker at the last.
Mickelson, two groups behind Els, parred the 17th after lagging another difficult downhill putt to a foot.
At the 18th, Mickelson ripped one down the fairway and left himself with 162 yards to the flag. He knocked his approach 18 feet over the flag but had to wait for playing partner Chris DiMarco to play out of a frontside bunker.
DiMarco failed to get out of the bunker in his first try, and ran his second blast to almost the exact spot where Mickelson's ball was. DiMarco putted first and gave Mickelson the line of the putt.
DiMarco missed his putt, but Mickelson studied the line as DiMarco's ball went left near the hole. Mickelson's observations paid off as he played his putt six inches left of DiMarco's and the ball rattled around before falling into the cup.
'There is still a feeling of disbelief but it feels awesome,' said Mickelson, who pocketed $1,170,000 for the victory. 'It is an amazing day, the fulfillment of all my dreams. I will remember that unbelievable back nine for ever and ever.'
Els fired a 5-under 67 to take second place. He was watching and preparing for a playoff when Mickelson sank the birdie putt.
'I played as good as I could,' said Els, who also took second in 2000. 'What more can you do? I guess Phil deserved this one. He played great down the stretch. He made birdie on 16. I heard that roar. And then obviously, I could hear from the crowd's reaction, he hit it pretty close on 18 and then he made a great putt there.'
K.J. Choi took third place at 6-under-par 282 after a final-round 69.
Sergio Garcia scared the leaders on the back nine. He played his final 12 holes at 8 under par for a 6-under 66 and a share of fourth place. Garcia matched European Ryder Cup captain and two-time Masters champion Bernhard Langer at 3-under-par 285. Langer briefly held a share of the lead Sunday, but fell off on the back nine and finished with an even-par 72.
Tiger Woods shot a 1-under 71 and tied for 22nd at 2-over-par 290. Woods is now majorless since the 2002 U.S. Open, but battled illness early in Sunday's round.
'I was feeling fine when I got here. I had a little sandwich and when I got on the range, I didn't feel too good,' said Woods, a three-time Masters champion. 'I am frustrated because I wasn't in there. It just wasn't there.'
It was there for Mickelson, although not early in the round.
He opened the final round tied for the lead with DiMarco, but tallied a birdie and three bogeys in a five-hole span from the third. Els knocked his second at the par-5 eighth to four feet and sank the eagle putt to move one ahead of Mickelson.
That's how the top of the leaderboard stood until Els made it to the par-5 13th. He hit another beautiful approach with his second and sank a 9-footer for eagle. The South African moved to 7 under par and took a three-shot lead over Mickelson.
Mickelson hit a solid tee shot at the demanding par-3 12th. His shot stopped 20 feet over the flag and he ran home the putt to cut the margin to two shots.
Mickelson had a look at eagle at the 13th but his 20-footer missed the hole. He kicked in the 3-footer for birdie and was back down a shot to Els.
Els drove into the rough on the par-5 15th, then flew the green with his second. He chipped to a foot and tapped in the birdie putt to go back up by two.
Mickelson responded yet again, this time at No. 14. His approach nearly tracked into the hole but he settled for an easy birdie and closed the gap to one stroke.
The left-hander could not birdie the 15th but it looked like Els might come back to him at 16. The South African was 40 feet left of the cup leaving him with a tricky downhill putt. He ran that six feet past the hole, but calmly made the par save to remain one ahead.
Mickelson then birdied 16 and Els made a routine two-putt par at the 17th. Els found a fairway bunker at the closing hole but hit a great iron shot to 25 feet. His birdie try missed left, leaving the door open for Mickelson.
Mickelson stormed through at the last for his second win of 2004 and his 23rd on the PGA Tour.
'All I wanted was an opportunity,' said Mickelson, who became the second left- handed player in a row to win this title after Mike Weir in 2003. 'I tried not to think too much about results. I was very fortunate.
'I will look forward to this first week of April every year the rest of my life.'
Mickelson fired a 31 on the back nine to win the title but Els played brilliantly as well. He posted a 3-under 33 on the final side but it was not enough to give him major No. 4.
'I'm very disappointed now, but I'll get over this,' said Els, who owns two U.S. Opens and a British Open. 'I feel like I'll win a major this year. I would have loved to have won this one. I'm chasing that Grand Slam a little bit in my career. But I'll have another shot, I'm sure of that, if I keep practicing, keep healthy, keep at it, I'll have another shot.'
Kirk Triplett, who along with Padraig Harrington, aced the 16th for a round of even-par 72. He tied for sixth with Davis Love III (70), Nick Price (70), Paul Casey (74), DiMarco (76) and former champions Vijay Singh (69) and Fred Couples (70). That group came in at 2-under-par 286.