He contested the Division of Melbourne Ports in 1975, but lost to the incumbent Labor member Frank Crean.
Following this, Johnston successfully sought Liberal preselection for Hotham, competing against nine other members.
[2] In 1977, he was elected to the Australian House of Representatives as the Liberal member for Hotham.
He was a member of the Parliamentary Yacht Club, along with Phillip Lynch, Alan Cadman and Marshall Baillieu.
[3] In 1979, Johnston was part of a delegation of Australian parliamentarians sent to Port Moresby to learn about resources development.