Career
Federal career
Johnston first ran for the Canadian House of Commons as a Social Cr candidate in the 1935 Canadian federal election. Johnston ran for re-election in the 1940 Canadian federal election. Johnston would increase his margin of victory winning his third term in office defeating four other candidates in the 1945 Canadian federal election.
He would be re-elected to a fifth term in office with another comfortable plurality in the 1953 Canadian federal election.
Johnston would suffer near defeat in the 1957 Canadian federal election winning by just 300 votes over Progressive Conservative candidate Eldon Woolliams. Parliament would be dissolved a year later, and this time Johnston would be defeated in a landslide, after facing Woolliams once again in the 1958 Canadian federal election.
Provincial career
After being defeated from federal politics he would run for a seat to the Alberta Legislature as the governing Alberta Social Cr candidate in the new provincial electoral district of Calgary Bowness. He was re-elected to a second term in office in the 1963 Alberta general election.
Johnston ran for a third term in office in the 1967 general election.
He was defeated in a closely contested race by Progressive Conservative candidate Len Werry. Johnston died on December 1, 1971.