The eighth rookie to win on the PGA Tour this season, Turnesa opened with rounds of 62, 64 and 69 at TPC Summerlin en route to a 24-under 263 total. He earned $738,000.
Turnesa birdied Nos. 9, 11, 12 and 14 to reach 25 under on the mostly sunny, breezy day. He also birdied the par-3 17th, but dropped a stroke on the par-4 18th.
Kuchar finished with a 64.
Former UNLV star Chad Campbell shot a 67 to tie for third at 22 under with John Mallinger (68) and 49-year-old Michael Allen (70). Tim Herron (62) and Davis Love III (67) followed at 21 under in the Fall Series event.
Kuchar, the second-round co-leader who began the final round at 19 under, birdied the par-5 13th and par-3 14th to take the lead at 24 under, but finished with four straight pars to fall a stroke back.
Turnesa, the 2007 Miccosukee Championship winner on the Nationwide Tour, also led after three rounds last month in the Viking Classic, but made a late double bogey and ended up losing to Will MacKenzie on the second hole of a playoff.
Turnesa is part of a famous golf family.
The 30-year-old former North Carolina State star's great-uncle, Jim, won the 1952 PGA Championship, and his grandfather, Mike, was a six-time winner on the PGA Tour who finished second to Ben Hogan in the 1948 PGA Championship.
Mike was one of seven brothers, six of whom played on PGA Tour. One of those, Joe, was a 15-time PGA Tour winner. The only brother who didn't turn professional was Willie, the 1938 and 1948 U.S. Amateur champion and 1947 British Amateur winner.