After a 28-foot birdie putt from Mickelson and a 13-footer from Bradley on the 18th hole at Riviera forced the playoff, Haas made his long birdie try for his fourth PGA Tour title.
The reigning FedExCup champion shot a two-under 69 in Sunday's final round to get in at seven-under 277. Mickelson, who was going for back-to-back victories after his win at Pebble last week, and PGA Champion Bradley both had even-par 71s.
"To beat a guy like Keegan and also Phil, guys of their caliber, in a playoff, is amazing," said Haas. "Something I'll never forget."
After three pars on the first extra hole, the same 18th where Mickelson and Bradley displayed magic putting touches, the trio was off to the drivable, but formidable, par-four 10th.
Mickelson went first and found the rough right of the bunker. Bradley landed in a similar spot, but in the trap and with the pin tucked on the right side, neither had much of a chance to keep it on the green, let alone get it close.
Haas drove into the left rough, but past the trees, offering him a decent look at the flag.
That was until he saw his lie. Haas had no chance to get it toward the pin with his ball half buried in the thick grass. He pitched sideways on to the green almost 40 feet short of the hole.
Mickelson tried a flop shot, but hit it too hard. His ball trickled off the green into sand on the left.
Bradley hit a great blast from the bunker that ran just 10 feet by the hole on the fringe.
Haas went first and poured in the long birdie putt to put the pressure squarely on the third-round co-leaders and good friends.
Mickelson hit a poor bunker shot and the ball stopped almost eight feet short of the hole. He was out and it was on the reigning PGA Champion's shoulders.
Bradley narrowly missed his birdie effort and the title belonged to Haas.
"I hit 30 putts, I maybe make five of them," said Haas. "I felt like I hit a nice putt. Trying to make a four, lucky to make three."
The very fact the championship made it to a playoff was remarkable.
Haas was in the clubhouse at seven-under par thanks to a two-putt birdie at the par-five 17th. He drove into the left rough at 18, but sank a tough five- footer for par to get in at minus-seven.
Mickelson and Bradley both fell a stroke behind Haas after bogeys at 15. Both parred 16 and 17, although Mickelson's 20-foot birdie try at the 17th came up just an inch short.
Haas told CBS' Peter Kostis in an interview he expected both Mickelson and Bradley to make birdie on 18, but statistics said otherwise. There were only six birdies when the last threesome arrived on the last this Sunday, but Mickelson and Bradley made Haas' words prophetic.
Both players hammered drives down the fairway. Mickelson hit an awkward wedge 28 feet right of the flag and just onto the fringe. Bradley played a great wedge to 13 feet.
Mickelson sank his long birdie putt drawing a thunderous ovation from the gallery.
"I kept fighting, and I was giving away shots and was trying to let it go and move on and see if I could capture one, and I finally got one to go on 18," said Mickelson. "It felt great. I kind of let out some emotion because it had been going on for so long."
Bradley followed suit, and while the applause wasn't quite as loud as Mickelson's, the outcome meant the three were headed back to the 18th tee.
"I'm very happy with the way I played," he said. "You know, playing with Phil in the final group and kind of holding my own and making one on top of him on 18 was something I'll never forget."
Bradley was in the best position on the first sudden-death hole. He had 11 feet for birdie, while Haas and Mickelson had four-footers for par. Both made their par attempts, then Bradley just missed his birdie putt for the win.
It was at No. 10 where Haas earned his fourth PGA Tour title and first since last year's Tour Championship. It was there in late September where Haas saved par from the water en route to a playoff win over Hunter Mahan and the $10 million FedExCup title.
"They were very different," Haas said of his two heroic shots in the last five months.
NOTES: Haas pocketed $1,188,000 for the victory...Mickelson was looking for back-to-back victories for the first time since 2006...Bradley hasn't won since the PGA Championship last year...World No. 1 Luke Donald struggled to a seven-over 78 on Sunday and fell into a tie for 56th at seven-over par...Next week is the WGC-Accenture Match Play Championship where Donald will defend his title and the Mayakoba Golf Classic, where Johnson Wagner won last year.