Two strokes clear when the rain-interrupted third round was completed earlier in the day, the American closed with a flawless five-under-par 67 for an 18-under total of 270.
A day after celebrating his 37th birthday, Bohn birdied 15, 16 and the par-five last, where he nearly holed out with a wedge approach, to complete a wire-to-wire victory.
Double U.S. Open champion Lee Janzen birdied the last for a 69 and fourth place at 13 under.
For much of a sun-splashed afternoon, the tournament came down to a three-way battle between Bohn, Overton and Merritt.
Bohn, who had taken a two-shot lead after completing the last 12 holes of his third round in the morning, was caught at the top after Overton birdied the first, fourth and fifth.
PGA Tour rookie Merritt, who covered the front nine in three-under 33, snatched the outright lead with back-to-back birdies from the 10th before faltering with a bogey on 14.
After knocking in the putt to edge a stroke in front, he rammed in a 22-footer on the 16th green to double his advantage.
"My caddie just kept saying 'Stay patient, stay patient,'" Bohn said after a tap-in for birdie at the last.
"I hit it to 20, 30 feet and couldn't really make anything but then I hit the shot on 15 and made a great putt on 16.
"I knew if I stayed patient, stayed within myself, it was going to happen. I'm thrilled."
Germany's Alex Cejka, who began the final round alone in second place, bogeyed three of the first six holes on the way to a 76 and a tie for 21st at seven under.
(Writing by Mark Lamport-Stokes in Los Angeles; Editing by Dave Thompson)