Smotherman, 32, carded a closing three-under-par 68 at the Black Desert Resort in Ivins to finish at 25 under.
That was good enough for the world number 121 to hold off Doug Ghim in second place, who shot a final-round 69 for a 24-under total.
David Ford’s seven-under-par 64 left him tied for third with Trace Crowe, three shots off the lead, with Kevin Streelman and Ben James a shot further back.
The round of the day came from Luke Clanton, who reeled off seven birdies and an eagle in a nine-under-par 62 that left him tied for seventh with Steven Fisk at 21 under.
The Bank of Utah Championship is part of the PGA Tour’s FedExCup Fall series, with elite players usually opting to skip the event after playing in the season-ending play-offs.