Trailing by a stroke with the back nine to go, Mickelson took advantage of several mistakes by Jeff Quinney to post a two-shot victory at the Northern Trust Open.
Mickelson shot a 1-under 70 and finished at 12-under 272.
It was Mickelson's 33rd PGA TOUR victory, but his first at Riviera. He played in the event 10 times, although once at Valencia Country Club, and once at the 1995 PGA Championship. When you're as good as Mickelson, there aren't many venues where success eludes you.
'I've wanted to win this tournament for a long time,' acknowledged the three-time major winner. 'It is pretty cool. Riviera is such a great golf course.'
Quinney had the longest active streak on TOUR without a three-putt, but snapped it en route to a bogey at 13.
Then he three-putted the 14th for a bogey.
Quinney bogeyed 15 and 17 as well to give Mickelson a commanding edge on the 18th tee. Quinney finished strong with a birdie at 18. He posted an even-par 71 to come in at minus-10.
Mickelson began the final round with a one-shot lead, but extended quickly thanks to a 12-foot birdie putt at No. 1. Quinney gave him one at the third with a bogey and Mickelson amassed a three-shot lead.
He would need all of it.
At the par-3 sixth, Quinney converted a 4-foot birdie putt to cut the margin to two. The eighth hole became crucial when Quinney sank a 16-footer for birdie and Mickelson missed his birdie effort from half that distance.
Mickelson now owned a one-shot lead and the par-4 ninth changed that. He drove into the rough and could not save par. Quinney made an 11-foot birdie putt at nine to move one shot clear of his fellow Arizona State Sun Devil alum.
Mickelson answered shortly after he fell behind. He rolled in a 7-foot birdie putt at 10 and both players birdied the par-5 11th. Mickelson had a tap-in, while Quinney made a 21-footer to stay tied.
Quinney hit his approach to 45 feet at the par-4 13th and might blame CBS for part of his problems. He had the longest active streak of no three-putts on the PGA Tour.
Until CBS put up the graphic.
Quinney recorded his first three-putt in 215 holes and the bogey dropped him one behind Mickelson. Quinney could not start a streak right away. He three-putted the 14th from 36 feet and Mickelson saved par from 8 feet to stay two clear.
Both players bogeyed 15 and neither converted makeable birdie chances at 16. Quinney missed a short par putt at the par-5 17th, essentially handing the title to Mickelson.
'It was a tough fight today because Jeff played so well,' said Mickelson. 'He gave a couple back coming in the end, but he played very well throughout the day.'
For Quinney, he will have to try again for PGA TOUR win No. 1, but summed up the differences between himself and Mickelson.
'The advantage Phil has is the extra length,' said Quinney. 'That's the advantage he has over me. It puts that much more pressure on my wedge game and short to keep up with his power.'
Padraig Harrington and Luke Donald both shot rounds of 3-under 68 on Sunday to share third place at minus-7.
Ryuji Imada (68) and Scott Verplank (70) tied for fifth place at 6-under 278.
K.J. Choi (70), J.B. Holmes (70), Robert Allenby (68) and Stuart Appleby (71) shared seventh at 5-under-par 279.