The light-hearted slug-fest is held each year prior to the year's final major, with prize money for the top three participants all going to charity.
American Jason Kokrak had no problems in competing and scooped top prize after smashing his driver 321 yards in wet conditions at Quail Hollow.
England's emerging star Tyrrell Hatton finished as runner-up, with a drive of 316 yards, just one yard ahead of Dustin Johnson in third.
Watson is known as one of the longest hitters on tour but was not playing ball and preferred to focus his attention on Thursday's opening round.
"I'm there to play golf, not to hit it far," he said. "I've got to practice. I've never been to the course. I don't need to worry about a long drive on the 10th hole."
Results of longest drive competition:
Jason Kokrak — 321 yards
Tyrrell Hatton — 316 yards
Dustin Johnson — 315 yards
Nicolas Colsaerts — 314 yards
Rory McIlroy — 313 yards
Justin Thomas — 312 yards
Brooks Koepka — 311 yards
James Hahn — 309 yards
Marc Leishman — 305 yards