The 55-year-old South African finished with a 4-under 68 at Fallen Oak for a one-stroke victory over 2011 winner Lehman and second-round leader Kevin Sutherland.
Frost was penalized after the coin marking his ball on the green moved when he accidentally dropped the ball on it. That left him with a bogey and cut his lead to a stroke.
''I marked the ball and as I picked it up, the ball just kind of slipped out of my hand, hit the coin and moved it,'' Frost said. ''I knew exactly where it was and I just moved it back and didn't think there was a penalty at all because I knew exactly where it was.
''There is some kind of rule that says in the act of marking the ball if you drop your coin. They told me I dropped the ball, which is an act of negligence and had to incur a one-stroke penalty. I was like, 'You've got to be kidding me. Last year disqualified and this year a one-shot penalty.'
''It was frustrating. You play by the rules and luckily for me in the end it didn't make any different and I'm happy Lehman didn't beat me in a playoff.''
Frost, disqualified last year for moving a stone in a bunker, made a testy-5-footer for par on the final hole to get to 10-under 206, then waited as Lehman missed virtually the same putt for birdie.
''I guess, just misread it,'' Lehman said about the putt that slipped by the right edge of the cup. ''I thought I hit a good putt. It certainly it didn't do what I thought it was going to do. I thought it was going to break left and it didn't.''
After playing the front nine in 1 under with a bogey and two birdies, Frost birdied Nos. 11-13 to take the lead and made another birdie on No. 15.
''Every victory you do accomplish holds a special place,'' Frost said. ''This is a very special victory for me.''
Lehman closed with a 70, and Sutherland had a 72.