Ian Johnston was born in Dublin, Ireland, but moved to Canada after World War II. He settled in British Columbia and became active in the local field hockey scene, joining Grasshoppers H.C. He was eventually selected for the Canadian national side, joining in 1962. He was a member of the team that travelled to the 1964 Tokyo Olympics, where Canada lost six of its seven classification round matches and placed joint-13th out of 15 squads overall. The Canadians had better luck at the 1967 Pan American Games, where they placed fourth after losing the bronze medal match against the United States; Johnston served as co-captain of the team. By then nearing the age of 40, Johnston retired from active competition shortly thereafter.