 In 1908 he was a crew member of the Leander eight, which won the gold medal for Great Britain rowing at the 1908 Summer Olympics.Johnstone had been commissioned a second lieutenant in the 3rd (Militia) battalion of the Black Watch in 1901, and was promoted to lieutenant on 11 October 1902.In 1909 Johnstone joined the Ceylon Government Surveys and in 1913 joined the Colonial Civil Service in Zanzibar.