A free spirit who dropped out of golf for almost 10 years and lived in his van for five, MacKenzie beat Marc Turnesa on the second playoff hole after Brian Gay dropped out on the first extra hole.
MacKenzie, who lost the third-round lead after a triple bogey on 18 that included a two penalty strokes, birdied three of the last four holes of regulation for a 4-under 68 to match Turnesa (70) and Gay (68) at 19-under 269 on the Annandale course.
Also the 2006 Reno-Tahoe Open winner, MacKenzie earned $648,000.
Turnesa took the lead into the par-4 17th, but dropped back with a double bogey.
Steve Allan (66), Bill Haas (68) and Casey Wittenberg (69) tied for fourth at 16 under.