Mickelson lost in a play-off at Riviera last year
FINAL ROUND LEADERBOARD
-12 P Mickelson (US)
-10 R J Quinney (US)
-7 L Donald (GB), P Harrington (Ire)
Phil Mickelson held off playing partner Jeff Quinney to win the Northern Trust Open by two shots at Riviera Country.
Mickelson, who had let slip an early three-stroke lead to trail by one after the turn, clinched his 33rd PGA Tour title with a one-under-par 70.
The American world number posted a 12-under total of 272 as Quinney imploded on the final nine.
Britain's Luke Donald shared third place at seven under with British Open champion Padraig Harrington.
Mickelson, making his 10th start at Riviera Country Club, follows in the footsteps of golfing greats Sam Snead, Ben Hogan and Byron Nelson.
I can taste where I want to get to but I'm not quite there yet
"The names that are on this trophy are pretty cool and bring up an element of history to this tournament," Mickelson said.
"Riviera is a special place and it has some of the greatest holes on the PGA Tour.
"Last year was really the first year I played well here and to break through with a victory feels terrific. I have wanted to win this for such a long time."
Mickelson has been working with swing coach Butch Harmon for almost a year and believes his game is close to where he wants it to be for this year's majors.
"I can see the improvement, but it's not quite to where I believe I can get it," he said.
"But it's been much better than in the past so I feel like I'm getting better. I can taste where I want to get to but I'm not quite there yet."
Donald, who had five birdies and two bogeys in his final round, was pleased with his first outing in America this year.
"It's my first event on the PGA Tour and to come out and to play solidly all week, it's very encouraging," he said.
"It's just showing the work I've been doing over the off season is starting to pay off."
Harrington, after dropping two shots on his outward nine, came home in 32 after four birdies helped him recover.
"I tried to be aggressive and go out there and maybe shoot a six or seven under par but that didn't happen," said the Irishman.
"It was a question of knuckling down and getting the best out of the round I could."