It wasn't his close call last year, when he bogeyed the final hole and lost in a playoff.
Nor was it the playoff loss two weeks ago in Phoenix, a sign that his game was on the right track.
Rather, it was a noise only Lefty could hear.
He switched golf balls this year to a softer cover for more spin, and figured he had made all the adjustments until he struggled with his speed on the greens at Pebble Beach last week, which held him back. That's when he decided to change the insert in his putter.
'When I had putted with the insert I had, it was a quieter sound when the ball was coming off and I couldn't hear it, and I was giving it a little too much,' Mickelson said. 'Consequently, my speed was going well by the hole. By putting in the firmer insert, I was able to hear it, and my speed and touch came back.
'Now I hear it and it feels great.'
The putter was key for Mickelson, who closed with a 1-under 70 for a two-shot victory over Jeff Quinney that gave him yet another PGA TOUR title on the Left Coast.
He now has 33 career victories, with 16 of them in California and Arizona.
But as much as the putter helped Mickelson, it went from a magic wand to a ball-and-chain for Quinney.
He made four straight putts, three of them for birdie, from outside 10 feet that took him from a two-shot deficit to a brief lead and ultimately to a duel alone the final seven holes. But Quinney again had trouble down the stretch.
He bogeyed three straight holes, starting with back-to-back par putts that he missed from 7 feet, that gave Mickelson a two-shot lead and some comfort as he played the final holes. Quinney lost all hope with a three-putt from 20 feet on the par-5 17th, and his 25-foot birdie on the final hole only made it look close.
He shot a 71 for his first runner-up finish in his two years on tour.
'I had two (putts) that I'd like to have back,' Quinney said. 'I just put a little too much pressure on the putter on the back nine.'
Mickelson, meanwhile, was solid throughout the week.
His putting kept momentum in his round of 64 on Friday to seize control, and in his 70 on Saturday to stay in the lead. And after a two-shot swing that gave Quinney the lead on the ninth hole Sunday -- Quinney made a 12-foot birdie, Mickelson missed the green well to the right and made bogey -- Lefty responded with clutch putts.
The first came at the 310-yard 10th hole, where Mickelson hit driver over the green and a flop shot to the skinny part of the green, the ball stopping 6 feet away. Quinney saved par with a 10-foot putt, and Mickelson made his on top of him to tie for the lead.
Mickelson pulled away when Quinney made the first of three straight bogeys, and the tournament turned on the par-3 14th.
Quinney went over the green and chipped 7 feet by the hole. Mickelson hit into a bunker and blasted out to the same distance, a few inches farther away. That meant he went first, and Mickelson poured it in for par.
Quinney missed his, the lead was two shots, the tournament effectively over.
Mickelson didn't make it a clean sweep of the West Coast Swing. He has never won in Hawaii, and only goes to Hawaii on vacation. He has never won the Accenture Match Play Championship, although he gets another shot starting Wednesday.
But he has won at every stop on the West Coast, from the ocean courses of Torrey Pines and Pebble Beach to soggy La Costa Resort to the desert tracks in Phoenix, Palm Springs and Tucson.
'I do enjoy the West Coast,' Mickelson said. 'I'm excited to play golf and I practice very hard on the West Coast when the season is coming around and I haven't played for awhile, I've got a lot of energy and I'm excited to get back out. I think all of these things, plus the fact that I grew up here and used to walk these fairways on the outside, I just have a great love for the West Coast.
'I've been fortunate to play well here.'
It should be no surprise that Riviera took so long.
Until last year, Mickelson had missed the cut four out of eight times, including the 1995 PGA Championship. He loved the look of Riviera, but was confounded by the sticky kikuya grass that could grab the ball as it was approaching the green.
There's an art to his course off Sunset Boulevard, and he was a slow learner.
'I didn't understand the nuances of this golf course, where you can and can't hit it,' he said. 'And learning those nuances and how to hit the shots into some of these greens has helped me over the years. Last year was when I started to put it together, and I'm fortunate to break through this year.'
Sweeter still is having his name on the roll call of a champions, a list that includes Ben Hogan, Byron Nelson and Sam Snead. And it's a list that doesn't include Tiger Woods, or even Jack Nicklaus.
And now that another victory is in the bag, he's hungry for more.
'It's not quite to where I believe I can get it, but I feel like it's been much better than in the past, so I feel like I'm getting better,' Mickelson said of his game. 'I can taste where I want to get to. But I'm not quite there yet.'