Howell closed with a 6-under 65 and got into a playoff when Mickelson bogeyed the 18th hole. Howell put away the two-time Masters champion with his third straight par save, holing a 3-foot putt on the 14th hole and raising his head to the sky in utter relief.
It was only the second victory of his career, and Howell had been haunted by nine runner-up finishes since winning the now-defunct Michelob Championship in the fall of 2002. He already had two runner-up finishes in four starts this year, including three weeks ago against Tiger Woods down the coast at Torrey Pines.
Mickelson, bidding for his second straight victory, had control throughout the playoff until coming up short of the green on the par-3 14th. He opted for putter, but it took a high hop leaving the blade and stopped 10 feet short. The par putt missed to the right.
Howell also was short, but his chip came out nicely just beyond the cup.
'I had every chance on the back nine to create some separation and not give anyone a chance,' said Mickelson, who twice missed putts inside 4 feet and closed with a 68. 'I felt like I had the tournament in my grasp and let it go.'
They finished at 16-under 268.
Ernie Els (67), Jim Furyk (67) and Robert Allenby (68) tied for third, three shots out of the playoff.
Els and Allenby both had chances to catch Mickelson along the back nine of a mostly sunny afternoon, but the Big Easy was tripped up by three bogeys, while Allenby fell back with a three-putt from 60 feet on the fringe at the 15th.
Howell earned $936,000 and is atop the PGA TOUR money list for the first time in his career. Aside from finally getting his hands on another trophy, Howell accomplished his first goal of 2007. The victory all but assures he can return to the Masters in April because the Augusta native will climb into top 25 in the world ranking.
Along the way, he exorcised a few demons.
It was four years ago at Riviera where Howell lost a three-shot lead, then missed a 5-foot birdie putt on the second playoff hole and lost to Mike Weir. This time, he got past No. 10 in the playoff with an unlikely par. He clipped the trees when he tried to chip off the cart path, then got up-and-down from about 80 feet for par with a superb chip with just the right pace.
And while putting has been his nemesis during his drought, he holed one big putt after another -- from 8 feet for par on the 18th in regulation to keep the heat on Mickelson, from 6 feet for par on the 18th to extend the playoff, and the 3-footer on 14 that must have looked like a mile.
'It's been a long time,' Howell said. 'I'm speechless.'
Coming off a five-shot victory at Pebble Beach where he tied the tournament scoring record, Mickelson got a couple of breaks that he thought would carry him to a comfortable victory.
His flop shot on the 10th was heading into the back bunker when a tuft of kikuya grass, cut like a Marine's flat top, stopped it on the edge. Instead of scrambling for par, he used a utility club to knock in a 20-footer for birdie and lead by two shots.
Then on the 12th, his approach bounced off Humphrey Bogart's tree -- the sycamore left of the green where the actor used to watch the tournament -- and caromed onto the green instead of down into a ravine. Those breaks were wasted, however, by missing two short putts and failing to make par on the final hole.
'I'll look back and see a lot of opportunities,' Mickelson said. 'On a good note, it's better to get those out of the way early.'
Howell trailed by as many as five shots early in the final round and was still four behind with eight holes remaining, seemingly playing for second place. But he pecked away, and his fortunes turned quickly when he knocked in a 30-foot birdie on the 16th, then two-putted from the fringe for birdie on the 17th.
In the group behind, Mickelson's momentum again was slowed by a short putt.
He missed a 2-foot par putt on the 13th hole for the second straight day, then failed to take advantage of a great shot on the par-3 16th. His 8-iron hopped onto the green and rolled to 4 feet, but the birdie putt stayed right of the cup.
Tied for the lead, Mickelson took it right back with a big drive that left him only a hybrid from 255 yards into 25 feet on No. 17 for a two-putt birdie. And he was presumably in good shape in the left rough on the 18th.
Howell kept his hopes alive by curling in his 8-foot par putt to post at 16 under, forcing Mickelson to make par to win.
Mickelson hit 8-iron from 204 yards, trying to get the ball to the front of the green and let it roll to the back, but it failed to clear a mound leading to the 18th green, and his chip came out flat to 18 feet. The putt never had a chance, sending both players back to the 18th for a playoff that lasted three holes and produced a huge win for Howell.
Asked which was more meaningful, the Nissan Open or '02 at Kingsmill, the 27-year-old didn't hesitate.
'This one,' he said. 'Because of the five-year gap between them. It's been a long, long time since I won a title.'
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