The pair ended regulation play tied at four-over-par 148 after Von Buynder signed for a 78 and McLachlan carded a 77 at Pretoria Country Club.
Von Buynder produced a superb four at the par-four 10th to seal his first national junior victory in extra time.
"I am absolutely thrilled that I got the job done," said the elated TuksSport Golf Academy player.
McLachlan reached 18 with a two shot lead over Von Buynder in regulation play, but squandered the advantage with a double-bogey.
Von Buynder responded with a well-timed six-iron into the green at the par-five and two-putted for birdie to force the play-off.
McLachlan split the 10th fairway and Von Buynder pushed his tee shot into the trees on the right.
"I thought it was all over, but I had a clean shot to the green, so I hit a lob-wedge over the trees and the ball finished about 25 feet from the pin," Von Buynder said.
"Keegan overshot the green with his second. He chip up, but the ball rolled back down and he had to chip again. I lagged my first putt close and tapped in for par while he putted for bogey.
"After the bogey, triple-bogey start I couldn't imagine that I would win, but I just kept grinding. It was quite tough out there, because it was bitterly cold and the wind was pumping.
"I kept my eye on the leaderboard through the round and I knew that I was keeping in touch. When we got to 18, I knew I had to make a birdie at the last. I am really pleased with the way I executed my plan for that hole to get into the play-off. Winning my first national title fighting like I did is awesome."
Southern Cape's Alex van Wyk carded a 73 to finish a further stroke back in joint third with Slade Pickering from North West, who closed with a 77.
KwaZulu-Natal's Luke Mayo (74), Kyle Barker (74) from Central Gauteng, Werner Deyzel (77) from Gauteng North and Ekurhuleni's Dylan Mostert (78) shared fifth on 150.
Von Buynder stole the show, but four other juniors were also given a reason to smile.
Mayo, Western Province's Luca Filippi, reigning Nomads SA Boys U-17 champion Herman Loubser and Wilco Nienaber from Free State were named to represent the South African Golf Association at the 2015 Italian International Under-16 Championship and Nations Cup in Italy in September.
The South African quartet will compete in the eight edition of the prestigious 72-hole international stroke play championship, which will be played concurrently with the team event at Golf Club Biella from 1-3 September.
