Johnston began his career in April 1951, at the Thompson Speedway, in a minor national sports car race in the United States, in a privately entered Jaguar XK120, with a trip to the podium, after finishing in third place.

[1][2] As a result of Johnston's success, he came into contact with Briggs Cunningham, and signed for his racing team.

Since the contract with Cunningham came into force in the summer, he raced a Jaguar D-Type of Art Feuerbacher, together with Bob Wilder to third place overall in the 12 Hours of Sebring.

Three weeks later, he had his first European success, when he partnered Briggs Cunningham to third overall in the 12 Hours of Reims, a race that didn't count towards the World Championship.

When the team returned to the US, Johnston started racing the first of Cunningham's Jaguar D-Types, they began winning immediately.

After the disaster of the 1955 Le Mans and the accident at Road America proved too much for Johnston to ignore and he retired after finishing third in a SCCA National race at Beverly Airport.

He was presented with the President's Cup for demonstrating ability, competitiveness and success in the SCCA National Championship.