Johnston was born in 1926 in Murray Harbour North, Prince Edward Island.[1] He married Mary Eileen Vanlderstine in 1948.[1] In the early 1950s and 1960s, Johnston fished and was co-owner of a lobster-packing operation, he travelled throughout the province selling farm equipment, operated a cattle and grain operation on the family farm in his home community, and was a carpenter.[1] In 1965 he purchased a small country store, which he operated with his wife and children, and two years later he opened a tent and trailer park.[1]
Johnston first attempted to enter provincial politics in the 1974 election, but was defeated by Liberal candidate Waldron Lavers.[2] Johnston ran again in the 1978 election, defeating Lavers by 49 votes to become councillor for the electoral district of 5th Kings.[3] Johnston was re-elected in the 1979,[4] and 1982 elections.[5] Johnston did not re-offer in the 1986 election.[1]