The 24-year-old from Sheffield beat Australian Marcus Fraser for his first European Tour victory at the BMW International Open in wet and windy Cologne.
Willett, a teammate of Rory McIlroy in the 2007 Walker Cup, feared it was going to be his 20th top 10 finish without victory when he threw away a three-stroke lead.
But the former world number one in the amateur ranks was given a reprieve when Fraser bogeyed the 456-yard final hole.
That meant a tie on the 11 under par mark of 277 after Willett’s superb three-iron escape from close to the trees and the pair had to play the 18th four more times to settle the issue.
First they parred it with Fraser holing from 12 feet, then they bogeyed it with Willett missing from four feet, then they parred it again.
Fraser, twice a winner on the circuit, was favourite to land the first prize of nearly £270,000 when he was 30 feet away in two and the Yorkshireman went over the green.
However, Willett almost chipped in, then saw Fraser lip out and miss his four-footer coming back.
“Amazing - it was a tough day,” said the world number 204, who today will be at Sunningdale trying to qualify for the Open Championship - as will Fraser.
“I’ve had some ups and downs in the last 18 months, but I’m injury-free now and I want to thank everybody for the support they’ve given me.”
One in front overnight, Willett birdied the third and fourth, but the script began to change when he three-putted the ninth.
Further dropped strokes came at the 11th and 15th, the pressure mounting after he had three-putted again for par at the long 13th.
With the weather at its worst he managed to par the final three whereas Fraser was in rough after his first and second shots down the last.
They were round in 71 and 73 respectively, but their work was from over in a tournament which last year saw a five-hole play-off before Pablo Larrazabal beat fellow Spaniard Sergio Garcia.
Joint-third only one behind were Ireland’s 45-year-old Paul McGinley after a 66, Spain’s Gonzalo Fernandez-Castano (69) and England’s Chris Wood.