Thirteen years on, and the 24-year-old is in the midst of a record breaking season and well on his way to fulfilling his dream of representing Britain on the international stage.
Last month, Johnston became the first Scottish fencer in 27 years to win the British Championship epee title. His success in London followed a string of impressive victories.
He opened 2016 by winning the Scottish Open for a third successive year and followed that triumph with wins in the Merseyside and Birmingham Opens. Now the coveted first GB cap is within touching distance.
“I’m pretty close,” he said. “Selection is based on ranking, I’m 17th at the moment but I’m due to jump up into the top ten by September. The big event that I am aiming for is the Grand Prix in Paris next spring.”
Back to that life changing afternoon at secondary school. “It was a sports day and I went along to try out fencing. The school arranged for Bert Bracewell (the legendary Scottish coach) to come along and I was hooked.
“It was when I went to Edinburgh University that it really took off. I was only selected for the B team, and I didn’t like it. It’s not what I wanted. So I started to work really hard and I rapidly improved.
“I think I’m a bit of a late bloomer. But fencers don’t usually reach their peak until they are about 30 so I’m lucky. I still have a long way to go.”
Now out in the workplace, Johnston operates as a computer programmer for a pharmaceutical company based in Stirling. So he now has to juggle his job with his sporting ambitions.
“I train four to five times a week, three of which are actual fencing sessions,” he said. “One is in Linlithgow and two are in Edinburgh.
“I’m a member of Edinburgh Fencing Club and it is a decent size and has a good membership. We train at Tollcross.”
He is delighted to report that there also remains a strong fencing club at his old school. In fact, he often returns to help maintain and boost the interest in the sport he loves.
“Apparently, it was my year that reached the highest level,” he stated. “I have been back a few times and I have refereed at the Academy during youth competitions.”
Fencing is an Olympic sport and Team GB have won five silver medals. Getting into an Olympic squad is another driving force for Johnston.
“I’ve got a way to go to make an Olympics, but I’ve got a good few years yet,” he said. “What I love most about fencing is that it is both a physical and mental challenge. It’s like chess on a stage.”