Piercy won his second PGA Tour title after firing a three-under par 67 in the penultimate group to post 17-under 263, but had to nervously watch as McGirt (69) bogeyed out of a share of the lead and Garrigus (70) left a 14-foot birdie effort short on the 72nd hole to finish at 264.
Josh Teater (66), Bud Cauley (66) and Chris Kirk (68) shared fourth, three off the pace at 266.
Former champion and world number one Vijay Singh joined two others in a tie for seventh at 268.
Piercy started two behind 54-hole leader Garrigus but flew out of the blocks with four consecutive birdies between the second and fifth holes.
But bogeys on the eighth and 14th holes gave McGirt the edge until Piercy made a short chip-in birdie on the 15th and McGirt three-putted the same green to tie things up at the top.
McGirt then pushed his approach on the last into a greenside bunker and couldn't get up and down, while Garrigus, needing birdie, managed a close approach, but left the 14-foot putt inches short.
Piercy now qualifies for the lucrative World Golf Championships-Bridgestone Invitational in Akron, Ohio, next week forcing him to miss out on defending his Reno-Tahoe Open title from 2011.
"I am so excited. Reno last year was definitely a platform for me to showcase how good I could really be and if that's any sign to how this year has been I've got some big things coming up," the 33-year-old Piercy said.
"You just have to believe in yourself. Half the battle is being confident in yourself. Just knowing you can get there. If you keep working hard, good things will come."
McGirt was left to rue letting a maiden title slip but still secured enough points to qualify for the upcoming playoff series and also his PGA Tour card for 2013.
"It was a struggle to get the butterflies to fly in formation, let's put it that way," he said.
"I have never been in this situation before. But to have the chance to win a golf tournament, to have the chance to get in a playoff, it was something new to me but it was an absolute blast. I just kind of hit the worst putt of the week on the last hole."
Graham DeLaet secured low Canadian honors at 278, 15 shots off the pace.