Mickelson captured his second major title with a score of four-under-par 276 after completing a weather-delayed final round of two-over 72 at Baltusrol Golf Club.
'This has been an absolutely amazing week,' said Mickelson, who earned $1,170,000 for the win. 'It has been so much fun.'
Bjorn birdied the 17th Monday, but missed the green long and left at the last. He could only save par to close a round of two-over 72.
Elkington hit poor drives off both the 17th and 18th tees. He recovered at the last as his tee ball bounced off trees left of the fairway and into the short grass. The Australian knocked his third to 12 feet, but missed on the left edge.
Bjorn and Elkington ended at three-under-par 277. Davis Love III, playing in the final group with Mickelson, parred his final seven holes to share fourth place with Tiger Woods at two-under 278.
Woods, a two-time PGA champion, did not have to play Monday. He was the leader in the clubhouse at two-under on Sunday, finishing a 68 before bad weather hit and forced the suspension of play.
The Monday finish was the first at the PGA Championship since 1976 at Congressional.
Vijay Singh, the 1998 and 2004 PGA champion, had a slim chance to get to minus-three, but missed a 10-foot par putt at 16. He also bogeyed the last to end at even-par 280.
Mickelson returned to the course Monday with a three-foot par putt on 14. He sank that putt, then two-putted for par from 18 feet on No. 15 to remain at minus-four.
The lefthander struggled on the par-three 16th, though. Mickelson's tee shot came up short in a front bunker. He blasted out, but could not save par from 20 feet away and slipped into a tie for the lead with Elkington at minus- three.
Elkington hit his tee ball into a fairway bunker at the par-five 17th. He recovered to drop his third shot on the green, but could only two-putt for par.
Bjorn, playing in the group between Elkington and Mickelson, calmly rolled in an 18-foot birdie putt on the 650-yard 17th to join the leaders at minus- three.
Mickelson knocked his third within 15 feet at 17, then missed a right-to-left breaking putt as it lipped out the right edge of the cup.
Elkington badly hooked his tee shot off the 18th tee, but got a tremendous break as the ball ricocheted off the trees into the fairway. He laid up with a fairway wood and dropped his third within 15 feet, but the birdie putt slid over the left edge to finish at minus-three.
'I think there is a lot to be said that the last guy out there has the final say,' said Elkington, who played in this event for the first time since 2002. 'Thomas and I both had good chances to birdie 18, but we couldn't do it. Phil hit a great shot and got his four.'
Bjorn knew he was still tied for the lead as he hit his second to the last. The Dane pulled the shot long and into the left bunker. He blasted out, but was left with some 25 feet for birdie. Like several other putts on this day, Bjorn's try lipped out and he tapped in for par.
'I came in here after a bad experience at the Masters on that Sunday and handled myself well,' said Bjorn, who shot nine-over 81 on Sunday at Augusta. 'I had a putt there on the last, that I think the whole world didn't think it could miss, but it did. That's just the way golf is. It's nice to be out on a Sunday and contend for a major championship. And you have to say that the best guy won this week.'
Mickelson, meanwhile, knew he what he had to do. Walking off the tee at the last, he saw that Elkington made par to end at three-under. He then watched as Bjorn also parred the hole.
The Arizona State alum found the fairway off the tee. Mickelson was near a plaque in the fairway commemorating one of Jack Nicklaus' two U.S. Open wins at Baltusrol. He tapped the plaque for good luck.
'To win here where Jack Nicklaus has won a couple of times, I even touched his plaque there on 18 to get some good karma,' Mickelson said.
Mickelson knocked his second just right of putting surface in some thick rough. He played one of his famous flop shots to two feet and kicked that in for birdie and the win.
'I knew that I needed to make birdie, I thought probably to tie, and then as I walked off the tee, I saw that Elkington had made par,' said Mickelson. 'I did not know what Bjorn had done, so I asked as I was coming up. When I hit the second shot I knew that I needed birdie to win, which was a nice feeling.'
Love, who struggled badly early on Sunday, was still in contention come Monday. At 15, he was 60 feet from the hole and two-putted for par. On 16, Love knocked his tee shot to the back fringe. He got up and down for his par. The 1997 PGA champion lost his tee shot right into the trees off the 17th tee.
However, Love had a clear shot and laid up nicely in the fairway, then two- putted for par from 30 feet out. Love found a fairway bunker off the tee at 18 and laid up. He knocked his third on the green and had a look at birdie, but could only two-putt for par to share fourth with Woods.
Woods bogeyed the first on Sunday. Then he lost his drive left into the trees and bogeyed the third. The No. 1 player in the game chipped in for par at the seventh after taking an unplayable lie as his drive came to rest under a tree.
The 29-year-old got one stroke back with a birdie on No. 8. Woods birdied 14 to get back to even-par for the day and for the tournament. He birdied 17 to get into red figures for the first time all week. The 10-time major winner got up and down for birdie for six feet out to close at two-under.
On Sunday, Mickelson sank a birdie putt at the fourth to get to minus-seven. Things fell apart from there though. He missed a six-foot par try on six and then missed another short par putt at seven to slip to minus-five.
Elkington moved to five-under and a share of the lead with an 18-foot birdie try on nine. Mickelson missed the green at the ninth and that led to a bogey, dropping him one behind Elkington.
Playing ahead of Mickelson, Elkington bogeyed 10 and slipped into a tie for the lead at minus-four. Mickelson then bogeyed the 10th to drop to minus- three, one shot behind Elkington.
Elkington extended his lead to two with a chip-in birdie at the 11th, but a big swing came at 13. Elkington three-putted for bogey, while Mickelson dropped in a 12-foot birdie putt to create another tie for the lead at minus- four.
The 1997 PGA champion parred 14, then missed the fairway at 15. Elkington was unable to save par from there to slip to minus-three. Mickelson missed a birdie putt on 14 before action was halted for the night.
A pair of U.S. Open winners -- Michael Campbell (69) and Retief Goosen (72) -- ended in a tie for sixth at one-under-par 279. They were joined there by Geoff Ogilvy and Pat Perez.
Singh tied for 10th at even-par with Steve Flesch, Dudley Hart, Ted Purdy and 2001 PGA champion David Toms.