On 20 May 1956, Johnston's world changed. He was part of the Cunningham team that raced at the Cumberland circuit, in their D-Types. They were joined by amateur driver Walt Hansgen. At the start of the race, Hansgen raced away and left the field behind to win, but Johnston failed to finish the race. A fortnight later, Hansgen would also be a member of the Cunningham team. At Road America, both Johnston and Hansgen rolled their D-Types. 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.