When he got hot with the putter in the Arnold Palmer Invitational, the American was simply unstoppable.

Scheffler made every important putt to build a three-shot lead at the turn and then poured it on with a game so complete he closed with a bogey-free, six-under 66 – the lowest score by two shots at Bay Hill in the final round – while playing in the last group.

He won the Arnold Palmer Invitational by five shots on Sunday (Monday AEDT), the largest margin at Bay Hill since Tiger Woods in 2012.

And it came a week before Scheffler defends his title at The Players Championship, which had been his last official PGA Tour win.

All that held him back over the past year was the putter. Scheffler switched to a mallet model this week, but far more important was keeping quiet between the ears, thinking more about the stroke than the outcome.

Both were superb. He made every putt inside 15 feet on the weekend to win at Bay Hill for the second time in three years.

"I just stayed in my own little space and tried to keep pushing." – Scottie Scheffler. 

U.S Open champion Wyndham Clark did his best at trying to stay close, and was two shots behind through eight holes.

But it all turned quickly. Clark drove into the rough and made bogey on the 9th, then didn't make another birdie until the par-5 16th on his way to a final-round 70.

Scheffler now has seven PGA Tour titles, all against some of the strongest fields. He picked up $US4 million ($A6.04 million) for winning this signature event, finishing at 15-under 273.

Shane Lowry, the former Open champion who started the final round tied with Scheffler, was off his game from the start. He had three bogeys in seven holes on his way to a 72 to finish alone in third.

Russell Henley and Will Zalatoris each had a 72 and tied for fourth. Zalatoris secured a spot in the Open for having the lowest score among players not already exempt.

Cam Davis was the best-placed Australian, in a tie for 18th place after carding a final-round 70 to finish the tournament at two-under.

Compatriot Jason Day finished tied for 36th at two-over after a final-round 73, and Min Woo Lee was tied for 44th after closing with a 73 to be three-over for the event.

Scheffler opened with a 12-foot birdie and no one caught him for the rest of the day.

"I just stayed in my own little space and tried to keep pushing," Scheffler said.

Former World No.1 Rory McIlroy, who started the final round four shots behind, shot 76 and tied for 21st place.

Victory should put a halt on questions about Scheffler's putting. He led the field in the most vital putting statistic on Sunday and ranked fifth for the week.