The 56-year-old Lehman birdied the par-4 15th and 16th, eagled the par-5 17th and birdied the par-4 18th for a 7-under 65 and a one-stroke victory over Joe Durant.
Lehman finished at 12-under 204 at Prestonwood Country Club and earned $315,000 for his ninth victory on the 50-and-over tour.
Durant closed with a 68. Needing to hole out from the fairway on the 18th to force a playoff, he hit close to set up his final birdie.
Bernhard Langer and Kenny Perry tied for third at 10 under. Langer shot a 68. Perry, the second-round leader, had a 70. Colin Montgomerie tied for fifth at 8 under after a 66.
Metropolis, IL native John Riegger finished tied for fifth at 8 under after a final round 69.