As part of the wartime political truce the other political parties did not contest what was a Liberal held seat and he was returned unopposed, succeeding Frank Kingsley Griffiths who had vacated the seat to become a high court judge. He had previously been MP for Willesden East (1923-24) and South Shields (1931-35). Johnstone was already a government minister despite being out of parliament, serving as Secretary to the Department of Overseas Trade. He held the seat until his death in 1945.