Johnstone joined Port Vale in March 1921 and made his debut on 25 March in a 1–1 draw with Barnsley at The Old Recreation Ground.[1] His brother John joined him the next month and the pair played two Second Division games together during the season.[1] He scored his first and only senior goal in a 1–1 draw with West Ham United at Upton Park on 23 April.[1] However, Harry lost his first-team place in August 1921 and was released at the end of the 1922–23 season, without having played since May 1921.[1]