He played junior hockey for the Toronto Marlboros and the Oshawa Generals, playing in the 1942-43 Memorial Cup Final with the Generals. Johnstone turned pro the following season, splitting his time between the Toronto Maple Leafs and the Providence Reds. In 1944-45 he played in 24 games for the Leafs, winning the Stanley Cup. He spent the next seven years in the minor pros. From 1953 to 1957 he played for the Sundridge Beavers who played in the Ontario Hockey Association's intermediate section. In the NHL Johnstone played in 42 games, all with Toronto, and scored 5 goals and 4 assists.