American Riley, who finished at 14-under 266, had no serious challengers on Sunday.
Keegan Bradley turned in a final-round 67 to tie with Scottie Scheffler (71) in second place. Collin Morikawa (68) placed fourth at 8 under.
Adam Scott finished as the leading Australian, tied 12th but 11 shots behind Riley after a 71, while Min Woo Lee slipped with a closing 74 to be tied 24th at one under and Cam Davis posted a 76 to tie for 56th at four over.
Despite 27-year-old Riley's breakthrough, there was a somber tone at the tournament just one day after PGA Tour golfer Grayson Murray, who was entered in the event and withdrew during the second round, died from suicide.
His parents released a statement revealing the cause of death, expressing their devastation and thanking golfers, the PGA Tour and others for their support.
"Super sad day in the golf world," Riley told the CBS broadcast. "My heart just goes to him and his family. There was definitely a little extra to play for today."
Golfers and caddies wore red and black ribbons on their caps or shirts on Sunday in memory of Murray, who often wore colours of his hometown Carolina Hurricanes of the NHL for Sunday rounds.
Riley's only previous PGA Tour victory came in the 2023 Zurich Classic of New Orleans, a team event with Nick Hardy as his partner. He was a runner-up when he lost in a playoff two years ago in the Valspar Championship.
This final round mostly lacked drama, with the margin reaching seven strokes after Riley's birdie on No. 11, one of four birdies on his day.
Bradley was within three shots until a bogey on the final hole, where he had posted par in each of the first three rounds. So Riley led by four with three holes to complete.
Maverick McNealy (66) was the only player to best Bradley's final round. McNealy tied for 17th.
Sepp Straka of Austria shot 70 and was the top international finisher, tying for fifth at 5 under with Mac Meissner (68), Pierceson Coody (74) and Hayden Buckley (74).
