The 24-year-old has just completed a hectic but successful 12-months, the highlight of which was winning the World Cup Series in Paris in August.
Now he has a long and anxious wait ahead of him. The New Zealand team is not expected to be named until six or eight weeks before the Olympics, which get under way in London on July 27.
"It has been an amazing 12 months, really," Johnstone said.
"I've done some really cool events and had some good results. Heading into the Olympics there is still a lot of room for improvement but I'm feeling like things are going pretty well and I'm fairly well on track for where I want to be in eight months' time."
There is some strong competition for the five available spots. Household names like Mark Todd and Blyth Tait are back on the campaign trail.
Todd won individual gold in the three day event at the 1984 and 1988 Olympics and picked up bronze medals in the team events in 1988 and 2000.
Tait won individual gold in 1996 to go with the bronze he won in the teams event that year, and at the Barcelona Games in 1992 he won individual bronze and silver in the teams event.
Andrew Nicholson, who shared in the New Zealand team success in 1996 and 1992, is also in the mix, along with Caroline Powell, Jonathan Paget, Lucy Jackson, Jonelle Richards and Neil Spratt.
Despite the competition, Johnstone said there is healthy sense of camaraderie as well.
"They used to be these famous people that I didn't really know, so that was a bit scary. But now that I know them reasonably well it becomes a lot less daunting. They have been a really big help, rather than scary competitors."
Johnstone has qualified for Olympic selection on two of his horses: Orient Express and KS Secrets. He hopes Incognito will be added to the roster after some recent strong performances. But qualification is no guarantee of selection.
"At this stage, I'm feeling pretty confident," he said.
"With horses, anything can happen and there is a long time to go. But I'm feeling good about having three horses qualified and don't have all my eggs in one basket.
"Orient Express would definitely be my first choice to ride there all going well. If he can continue on his current form ... I think we'll be in with a pretty good shot."
Jonhstone has had the 13-year-old for the past seven years and he admits he has a soft spot for the animal.
"He is a bit of a pet, really. He is a lovely horse. We know each other inside out, so that really helps."
Born in Dunedin and raised in Outram, Johnstone moved to England at end of May to chase his Olympic dream and is based in the "beautiful" Cotswolds, in Gloucestershire.
"The move has had its challenges but has also been really interesting as well."
He lives just down the road from Princess Anne and has even had a brief chat.
"I've met Zara Phillips and shook Princess Anne's hand and had a wee chat. That was nice. She asked where I was living in England and I told her that I just lived about 5min away from her. She is just about my neighbour, actually."
While Johnstone, who was part of the bronze-medal winning team at the World Equestrian Games last year, might be rubbing shoulders with royalty, the riches are yet to come.
"There is money to be made at the big shows but it is pretty hard to get it. It is an expensive sport but I'm lucky to have good support from the equestrian high-performance programme."
Johnstone will be based in Dunedin until he returns to England in late January, to prepare for the beginning of the eventing season which gets under way in March.
Clarke Johnstone
• Age: 24
• Born: Dunedin
• Educated: John McGlashan, University of Otago
• Member of New Zealand team which placed third at the World Equestrian Games last year
• Won the Eventing World Cup Series in Paris in August
