The 22-year-old, ranked 1,207th pre-tournament, was fourth reserve at Quail Hollow and only got his place due to other players withdrawing.
He birdied the 72nd hole to force a playoff, before clinching the title when Lynn found the bunker with his approach at the 18th.
"This feeling is unbelievable right now," BBC Sport quotes the American as saying. "Winning was not on my mind [approaching the 18th].
"I hit a six-iron and choked up a little bit and hit a draw in there. I was trying to hit it as close as I possibly could.
"I knew Dave [Lynn] was at eight. I was just trying to get tied with David."
Overnight leader Phil Mickelson bogeyed the final two holes to finish on seven-under par, one shot behind the leaders.