It capped off a final round of lustrous cheers in thin air that included Thomas holing a wedge for eagle on the final hole of regulation.

Mickelson, who closed with a five-under 66, won for the first time since the 2013 Open Championship at Muirfield, a stretch of 101 tournaments worldwide.

"I can't put into words how much this means to me," Mickelson said.

"I knew it was going to be soon; I've been playing too well for it not to be.

"But you just never know until it happens."

Thomas was coming off a play-off victory at the Honda Classic last week, and he delivered the biggest moment at Chapultepec Golf Club.

Tied for the lead, his shot to the 18th green from 119 yards landed in front of the pin, bounced past the pin and spun back into the hole for an eagle.

It almost was too good to be true. Thomas, who said Thursday he had never felt worse over the ball, had a 62-64 weekend – 16-under par for the two days – and suddenly had a two-shot lead.

Mickelson broke a 1,687-day winless streak by clinching the WGC-Mexico Championship. PHOTO: Gregory Shamus/Getty Images.

Mickelson, the oldest player in the field at 47, responded with a two-putt birdie on the par-5 15th and a 20-foot birdie putt on the 16th to tie Thomas at 16-under 268.

Tyrrell Hatton, playing in the final group with Mickelson, was stride for stride.

He capped off a 3-3-3-3 stretch on the back nine with an eagle at the 15th.

But on the final hole, the Englishman missed the green to the right, chipped 10 feet by and missed the par putt for a 67 to fall out of the playoff.

The sudden-death playoff – the sixth in eight PGA Tour events this year – didn't last long.

Thomas went long on the par-3 17th hole and chipped to just inside 10 feet.

Mickelson's 18-foot birdie putt for the victory swirled around the cup, more agony for the five-time major winner who has seen plenty of it since his last victory.

Thomas fell just short of back-to-back victories after failing to make his up-and-down on the first playoff hole. PHOTO: Rob Carr/Getty Images.

Thomas, however, never got his par attempt on the right line.

Mickelson won his 43rd PGA Tour event and, just a month after being on the verge of falling out of the top 50 in the world for the first time in two decades, moves to No.18 in the world.

Shubhankar Sharma, the 21-year-old from India who started Sunday with a two-shot lead, didn't make his first birdie until the 12th hole and finished with consecutive bogies for a 74, six shots behind in a three-way tie for ninth.

Hatton tied for third with Spaniard Rafa Cabrera Bello (67), who holed a bunker shot for eagle on the opening hole and was among six players who had at least a share of the lead.

Adam Bland closed with a 69 to be the best-placed Australian in a tie for 18th at seven under. Marc Leishman's 77 sent him tumbling down the leaderboard to tied 37th while Wade Ormsby (70) finished tied 50th and Brett Rumford (73) 62nd.