Willett began the day in the joint lead on 12 under with Matt Fitzpatrick but clinched victory by one shot ahead of his countryman on 17 under overall thanks to a bogey-free final round at the picturesque Crans-sur-Sierre course in the Swiss Alps.
The 27-year-old Sheffield golfer, who finished in a tie for sixth at the Open last Monday, picks up his first European Tour victory since triumphing at the Nedbank Golf Challenge in South Africa in December.
Tyrrell Hatton completed an all-English top three when he carded a best-of-the-day 62 to finish two shots off the pace on 15 under, with Sweden’s Pelle Edberg (14 under) and India’s Anirban Lahiri (12 under) fourth and fifth respectively.
Willett said: “Fitzy played some great golf, kept the pressure on all day. Right up until the last me and (caddy) Johnny (Smart) said, ‘Be prepared just in case — stranger things have happened in golf’.
“But I’m delighted. A great day today, I can’t be happier.”
Meanwhile Olivia Mehaffey left her best until last in the European Ladies Championship at Murhof in Austria, but still finished well outside the medals.
The 17-year-old international from Royal County Down posted a four under par final round of 68 for a 72-hole aggregate of 286 which gave her a share of 24 th place.
But that was a yawning 13 strokes behind new champion Luque Parra from Sotogrande, who triumphed by two strokes from Bangalore’s Aditi Ashok.
Mehaffey, the current Welsh and Scottish Open Champion had halves of 33 and 35 birdieing three holes on the bounce from the seventh.
Her closing 68 followed rounds of 70, 77 and 71.
None of the other Irish players made the 54-hole cut.
Elsewhere, in the European Young Masters in Switzerland, both Thomas Mulligan and Mark Power finished with totals of 231 to share 32nd place while in the girls event, Annabel Wilson ended on 230 in joint 21st spot, and Mairead Martin was 36th with a 237 aggregate.