Theegala fends off Thomas to claim maiden PGA Tour victory in California
Theegala, who started the final round sitting on a two-shot lead, never looked like squandering that advantage during a composed final round on the Silverado Resort's north course in Napa.
Theegala from California carded a four-under-par 68 including seven birdies to finish on 21 under for a comfortable win with a crowd of close family and friends looking on from the galleries.
Theegala's procession to the title was effectively sealed on the 14th hole, when he rolled in a superb 20-foot birdie putt to pull five shots clear.
South Korea's Kim Seong-Hyeon (25), playing in the group ahead, raised hopes of a rally with back-to-back birdies on the 15th and 16th holes to trim Theegala's lead to three shots, but the leader would not be denied.
Theegala responded to Kim's challenge by drilling a superb second shot from the fairway to seven feet of the hole on the par-four 16th. He then coolly rolled in the birdie putt to take a four-shot lead with just two holes remaining.
A solid par on the par-three 17th left Theegala four clear heading to the par-five 18th, and although Kim bagged another birdie on the last to cut the lead back to three, the outcome was never in doubt.
Theegala played conservatively to close with a bogey for an aggregate of 267.
"It doesn't feel real, it's probably not going to sink in for a while," an elated Theegala said after the win.
"But man, there was a lot of good golf, and that was probably the most fun I've had on a golf course in my life. Family and friends cheering me on, it's mind-blowing.
"This feeling is incredible and I couldn't have done it without my whole team and everyone out here. It's a team effort and for me to put it together like this means the world."
Kim finished on 19 under after a closing four-under-par 68.
Australia's Cam Davis (28) finished alone in third on 17 under after a closing 70, one clear of Eric Cole (35) on 16 under, who also finished with a 70.
Two-time major champion Justin Thomas' (30) challenge faded in the final round with a level-par 72 to leave him in fifth on 15 under, six off the lead.
Brendon Todd (38) finished in sixth on 14 under after moving up the leaderboard with a six-under-par 66 to close.
The day's biggest mover was Troy Merritt (37), whose seven-under-par 65 was good enough to propel him 30 places up the leaderboard into a four-way tie for seventh on 13 under.
Merritt finished alongside Max Homa (32), who closed with a three-under-par 69, England's Callum Tarren (32) and US veteran Matt Kuchar (45).
