After the war, Johnston worked as an underwriter for Manufacturer's Life Insurance Company, then from 1948 to 1952 at the Portage la Prairie Post Office and then operated his own grocery store.
[2] The once-dominant Liberal Party was the official legislative opposition for most of the 1960s, and its support dropped even lower in the election of 1969.
The party won only five seats, and Johnston was re-elected[1] over Tory candidate Harvey Carmichael by only eleven votes.
After the election, he offered to serve as Speaker of the legislature to allow Edward Schreyer's New Democratic Party to form a minority government.
Johnston increased his margin of victory to 36 votes in the election of 1973, over Tory George Fairfield.