He was a Labor member of the New South Wales Legislative Assembly, representing Cobar from 1965 to 1968 and Broken Hill from 1968 to 1981.
He was educated at public schools in the area and studied mine draughting and engineering.
In 1936, the year he joined the Labor Party, he was employed with the North Broken Hill Mining Company, and became a member of the Broken Hill Miners' Union.
In 1941 he enlisted in the AIF, serving in Tobruk, Syria and El Alamein.
[2] Cobar was abolished in 1968 and Johnstone successfully transferred to Broken Hill.