The American golfer edged out Justin Thomas in Sunday's playoff round at the WGC-Mexico Championship to earn his first victory since the 2013 British Open at Muirfield.
It has taken 101 tournaments over the course of almost five years, but the 47-year-old never doubted himself for a second.
The oldest man in the field is now the oldest ever winner of a World Golf Championship event.
"I don't know what to say," said Mickelson. "It's been a tough go the last four years not playing my best, but to have the belief that I was going to get there and to finally break through and do it was incredible.
"I believe that more is to come, and I feel like I'm starting to play some of my best golf. To culminate here in Mexico City with a World Golf Championship victory is exciting to me beyond words because it just validates the hard work I put in and the struggle for the last few years to finally get my game back."
That Mickelson was able to get the better of Thomas -- winner of seven PGA Tour titles in his last 32 starts -- is testament to the magnitude of his return to golf's grand stage.
Thomas, the Fedex Cup leader, played his final 36 holes in 16 under, tying the all-time weekend record on the PGA Tour.
"If you'd told me I'd be in a playoff after two rounds, I would have told you you're lying," said Thomas, who would have become the first player since David Duval in 1997 to win consecutive PGA Tour events in playoffs.
"But I played unbelievable golf these past two days. I'm probably more proud of myself than I ever have been."
Englishman Tyrell Hatton and Spaniard Rafa Carbera Bello were a shot back at 15-under in third position. While both have three victories on the European Tour, their wait for a maiden PGA Tour victory goes on.
Indian 21-year-old Shubhankar Sharma had led through 54 holes on his WGC debut but ultimately fell back to 10-under-par.
For Mickelson, the victory was a long time coming but the signs have long been good. The five-time major winner traveled to Club de Golf Chapultepec in the city of Naucalpan this week on the back of three consecutive top-six finishes.
Through the tough times and near misses, he has never lost hope.
After tying for fifth at the Waste Management Phoenix Open last month, a bullish Mickelson said "I think that my game's getting better each week."
Later that month, having tied for second at the Pebble Beach Pro-Am, he told reporters "I'm optimistic about the year."
A week later, at the Genesis Open, he proclaimed "It's going to happen soon."
So it has proved.
Mickelson closed with a five-under round of 66 and got the better of Thomas after one playoff hole to win his third WGC title. Only Dustin Johnson (five) and Tiger Woods (18) can boast more.
"I said earlier that if I couldn't win, I want Phil to win, and I stand by that," said world No. 2 John Rahm who ended up at six-under, tied for 20th.
"This is an emotional win for him, and it's emotional for me, too. He's not just one of the greatest players and personalities, he's one of the greatest people ever to play the game."
"Congrats Phil Mickelson! From Murifield to Mexico, it's great to have you holding a trophy again," tweeted Jack Nicklaus. "Fantastic event, fantastic time for golf and the PGA Tour!"
"What a week," posted Thomas. "Easy to be upset about losing the playoff, but can't believe I even had a chance! Hats off to Phil, dude has grinded (sic) his ass off the last few years to be back in the winner's circle. Happy for him."
The big 50?
Mickelson now has 43 PGA Tour victories and looks to be back to his best with the Masters on the horizon.
Asked if he can reach a half century of wins, "Lefty" answered with steely conviction: "Oh, I will."
He'll have to rack up a further seven titles but the Californian knows exactly what he's capable of.
"I know how great the young players are and I appreciate their skill level," said Mickelson. "I also know the level I'm able to play, and I'll get there."