Adamonis and Van der Walt played six playoff holes head-to-head after Ron Whittaker was eliminated on the first sudden-death hole and Vance Veazey was knocked out on the second.
The four players ended regulation tied at 10-under 278.
Adamonis won the second-longest playoff in Nationwide Tour history by making eight consecutive pars -- the last one coming on a 4-foot putt at the par-3 16th to finally finish off Van der Walt.
'I don't care -- any way I can get a win is great,' said Adamonis. 'I just kept on grinding. That's kind of the way I've always played my game. I just hung in there and finally got a break in my career.'
Adamonis shot a 2-under 70 in the final round at Midland Country Club, while Van der Walt and Veazey both had 4-under 68s. In the best position overnight, Whittaker managed only a 71 on Sunday to join the playoff.
Down the stretch, Adamonis and Veazey both collected key birdies at the 14th, 15th and 17th holes. But Adamonis bogeyed the 18th to miss a chance to win in regulation.
'The last three holes in regulation, I was very nervous,' said Adamonis. 'It's tremendous for me to get a win because I won't be as nervous (in the future), and I can play some better golf.'
Whittaker made a birdie at the 17th to reach 10 under, while Van der Walt got to 11 under with a birdie at the 15th only to drop a stroke at the 16th and join the leaderboard logjam.
Brian Guetz, tied for the overnight lead and playing in the final group, was the last player with a chance to win in regulation. He was 10 under par heading to the 18th, but closed with a bogey to join four other players at 9-under 279.
Guetz (75) shared fifth place with Jim McGovern (67), Garth Mulroy (69), B.J. Staten (71) and Jon Mills (71).
Greg Chalmers, who shared the third-round lead with Guetz, stumbled to a 5-over 77 on Sunday and dropped all the way into a tie for 15th place at 7-under 281.