Young played bogey-free on the back nine where so many hopes faded, and blistered a 375-yard drive — the longest ever on the 18th at the TPC Sawgrass — that set up a par and Sunday's four-under 68.
It was only his second victory on the PGA Tour. He tied the tour record with seven runners-up finishes before finally winning late last summer in the Wyndham Championship. But this is the PGA Tour's crown jewel, loosely known as the fifth major, and the pressure was just intense at the end.
"The nerves kicked in over the eight-inch putt on the last," Young, who moves to world No.4, said.
"That hole looked really, really small there from pretty close range. So happy to have finished it off, and just really excited to have played the way I did."
Fellow American Xander Schauffele finished third at 11 under and Scot Robert MacIntyre fourth at 10 under after their final-round 69s.
Min Woo Lee (74) was the best-placed Australian in a tie for 32nd at two under, while Adam Scott (75) finished two over and Jason Day (80) four over.
Ludvig Aberg, who had a three-shot lead going into the final round, imploded on the back nine with shots into the water on consecutive holes. He carded a 76 and tied for fifth.
"I would have loved to be standing where Cameron is standing right now," Aberg said.
"It definitely stings a little bit."
Englishman Fitzpatrick was the first to seize on the Swede's collapse, hitting wedge to tap-in range for birdie on the 12th and a tee shot to four feet for birdie on the 13th.
Young stayed with him and caught him with the clutch birdie on the par-3 17th. A day earlier, Young found the water on the 18th and had to scramble for double bogey. This time, he drilled his tee shot and hit a lob wedge from 98 yards to the back fringe.
When Fitzpatrick missed his putt to force a playoff, Young was left with a tap-in par to finish at 13-under 275 and collect the $US4.5 million ($A6.4 million) prize.
With AAP.
