The Dumfries & County player claimed Scottish golf’s latest title triumph in the Rainbow Nation following a last round full of twists and turns.
Johnston and Drumoig’s Connor Syme both started the day two shots behind newly-crowned South African champion Christo Lamprecht.
The lead soon changed hands as Johnston started strongly and, as Syme dropped four shots in five holes, it looked as though it had become a two-horse race.
Johnston then handed the advantage back to Lamprecht by running up a triple-bogey 7 at the 11th only for the young South African to take back-to-back double-bogeys at the 13th and 14th.
As Syme recovered with three birdies in five holes, the trio then found themselves fighting it out for the title down the stretch.
Having made back-to-back birdies at the 14th and 15th, Johnston held a one-shot lead with three to play over both Syme and Lamprecht.
A par-par-par finish did the trick, closing with a one-under-par 71 for a title-winning eight-under 280 aggregate.
“I’m absolutely ecstatic,” said Johnston. “I feel like I finally got the money off my back. I won a lot of team events when I was at the University of Tennessee, but I never won an individual title, although I came close so many times.
I can’t begin to describe how amazing it feels to land this victory.
“I hope this pays back all the support my family and my coach James Erskine has given me and the trust Scottish Golf, Aberdeen Asset Management, Dunhill and Mr [Johann] Rupert placed in me to bring me out with the national squad to South Africa this year.”
Syme, last year’s Australian Amateur champion, finished a shot behind in second, with Old Ranfurly’s Jamie Stewart making it a Scottish 1-2-3.
That followed Lamprecht running up a double-bogey 7 at the last, which Stewart had birdied earlier for a closing a 71.
Adding to a good week for the Scottish contingent, Peebles player Craig Howie finished fifth while Calum Fyfe (Cawder) and Eric McIntosh (Bruntsfield Links) also ended up inside the top 15.
Johnston’s victory came a week after he’d reached the quarter-finals in the South African Amateur Championship at Humewood.
The University of Tennessee graduate had also helped Scotland retain the team title over the first three three rounds at Leopard Creek.
Johnston’s success adds to South African Amateur title triumphs by Michael Stewart, Brian Soutar, Daniel Young and Craig Ross in recent years.
David Law also won the Northern Amateur Open on one of the first trips made to the Rainbow Nation by Scottish Golf.