Piercy matched the tournament record of 17-under-par 263 thanks to his final round of 3-under 67, tying a mark set by Johnny Palmer in 1952.
But he had to wait until William McGirt bogeyed the final hole at Hamilton Golf & Country Club to celebrate. McGirt (69) ended one stroke back at minus-16, tied with Robert Garrigus (70).
The victory gave Piercy a spot in next week's WGC-Bridgestone Invitational.
Bud Cauley and Josh Teater each fired 4-under 66s to finish knotted in fourth place with Chris Kirk (68) at minus-14.