That saw him open up a three-stroke lead heading into the final round.
Dunlap, who won the 2023 US Amateur, then kept his composure on the PGA Stadium Course to become just the eighth amateur to win on the PGA Tour since 1945.
He carded a final-round 70 to see off South Africa's Christiaan Bezuidenhout by one shot on a tournament record score of 29-under.
Mickelson had been the last amateur to win a PGA Tour event when he captured the Northern Telecom Open in Houston in 1991.
Dunlap becomes the second youngest amateur to win on the PGA Tour and the youngest in 93 years, as well as the youngest ever champion of The American Express, which used to be the Bob Hope Classic.
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Dunlap and Tiger Woods are the only two golfers to have won an US Amateur, US Junior Amateur and PGA Tour title.
As a result of his maiden PGA Tour victory, Dunlap vaults from 4,129th in the Official World Golf Rankings down to 68th!
Rahm, who won The American Express in both 2018 and 2023, was mightily impressed with what he saw from Dunlap.
The latest and biggest LIV Golf League signing to date took to Twitter immediately after Dunlap drained a six-foot par save on the 72nd hole to hold on for a one-shot win over South Africa's Christiaan Bezuidenhout.
Rahm believes Dunlap has one of the brightest futures in the game.
Rahm was tweeting at a time when he is still working hard to build his new LIV Golf team ahead of the 2024 season, which starts in less than two weeks at Mayakoba in Mexico.
It is understood Rahm is going to sign 19-year-old amateur Caleb Surratt.
But after watching the PGA Tour action on Sunday night, the Spaniard might be wishing he had another amateur golfer up his sleeve.
It is also anticipated Kieran Vincent will link up with Rahm's team, a player who earned his playing rights on LIV Golf after coming through their recent Promotions event.
A report emerged in The Telegraph over the weekend that Rahm is trying to persuade his European Ryder Cup teammate Tyrrell Hatton to come and join him on LIV Golf.
Rahm's team name is expected to be called Legion XIII.
But it's not been confirmed yet.
Rahm was not the only LIV Golf star to tweet a message of congratulations to Dunlap as HyFlyers captain Mickelson also got involved.
Mickelson, who had been the last amateur to win a PGA Tour event, tweeted:
It now remains to be seen what decision Dunlap makes surrounding his golf career.
As a result of his amateur status, he does not receive any money, nor does he receive any FedEx Cup points.
But the win most certainly opens up plenty of doors for him.
Dunlap won the 2023 US Amateur, as well as the 2021 US Junior Amateur.
Only he and Tiger Woods are the only two golfers of achieved that feat.
As a result of being the reigning US Amateur champion, Dunlap will earn a place in The Masters, The Open and US Open this season should he maintain his amateur status.
Should Dunlap make the decision to turn professional this season, he would be out of The Open at Royal Troon.
He would get a spot in the US PGA though, and regardless of whether he turns pro or not, he earns an invite into The Masters.
Dunlap will also have starting berths in all of the bumper prize purse signature events on the PGA Tour for the remainder of 2024 if he turns pro.
He would become fully exempt on the PGA Tour for two years, too.
Dunlap therefore has plenty to ponder over the coming days and weeks.
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