Johnson was first elected to the Manitoba legislature in 1958, for the riding of Gimli, north of Winnipeg. A Progressive Conservative, he was appointed Minister of Health and Public Welfare in the minority government of Dufferin Roblin, who had personally recruited him to run for the party. He retained the health portfolio when the Progressive Conservatives won a majority government in 1959, and oversaw a policy of major hospital expansions in the province and other significant reforms between 1959 and 1963.