It was the first time this year anyone has led a tournament wire-to-wire.
Flesch, who began the day with the same lead after rounds of 63-69-69, had three birdies and three bogeys to finish 15-under par 273. He missed only 10 greens his first 54 holes but hit only three of his first 10 on Sunday with winds gusting up to 30 mph at the 7,472-yard Montreux Golf & Country Club on the edge of the Sierra Nevada.
Kevin Stadler shot a 70 and Charles Warren a 71 to tie for second at 10-under 278. Rich Barcello, who lives in Reno and played golf at the University of Nevada, had a 68 for the best round of the day and a tie for fourth with PGA TOUR rookie John Merrick (74) at 8-under 280.
Steve Elkington bogeyed the last hole for a 73 to finish another stroke back at 7-under 281 in a tie with Shaun Micheel (68) and Brendon de Jonge (72).
The $540,000 winner's check gives Flesch $946,586 for the year and pushed his 10-year career earnings past the $14-million mark. It's only the second time this year he's finished in the top 10, tying for fifth last month at the U.S. Bank Championship in Milwaukee.
The mountain winds swirling through towering pines kept the challengers from taking advantage of Flesch's slow start, which included two bogeys in a 37 on the front nine.
The 40-year-old left-hander also bogeyed the par-4 10th when his approach came up short in a bunker. But on the 584-yard 11th, his third shot from 95 yards nearly bounced in the cup and he tapped in for birdie.
He parred the next six holes, getting up and down from the rough behind the green on the 491-yard, par-4 14th and barely missing a 14-foot birdie attempt on the 477-yard 15th. He drove the ball 364 yards in the middle of the fairway on the 636-yard 17th, but hit his approach in a deep trap in front of the green before hitting out to 28 feet and rolling in the birdie putt, then two-putted from 13 feet on the 18th for par.
The victory ensures Flesch a spot in next week's PGA Championship for the 10th year in a row. He's made the cut there seven of the past nine times, his best finish was a tie for 10th in 2005.
In addition to leading from start to finish, Flesch's five-stroke lead heading into Sunday's final round was the biggest on tour this year, eclipsing the four-stroke lead Tiger Woods had after 54 holes at the World Golf Championship.