At the 1981 World Rowing Championships, the eight missed the A final and came seventh overall. At the 1983 World Rowing Championships in Duisburg, Germany, he was a member of the coxed four (alongside Conrad Robertson, Keith Trask, Les O'Connell, and Brett Hollister as cox) and won gold. At the 1984 Summer Olympics in Los Angeles, Johnston was again member of the New Zealand eight and they entered the competition as the favourite, with their strongest competitor, East Germany, absent due to the Eastern Bloc boycott, the Kiwis were the favourite to win the event. In the end, the eight finished a disappointing fourth and missed out on a medal.