He attended the Sheffield Peace Congress in Warsaw in 1950 and then embarked on a brief tour of Moscow and Leningrad, which resulted in the cancellation of his passport.
In 1951, he gave up his practice to work full-time as a communist organiser, and in 1954, he was elected to the South Australian state committee of the party.
Johnston disapproved of the excesses of both Stalinist and Maoist communism but remained committed to the ideal; he was put forward as Queen's Counsel (QC) in 1969 but was rejected on political grounds.
[1] Throughout the 1970s and 1980s, Johnston's law firm, which he ran with his wife Elizabeth and their partner Robyn Layton, dealt with a variety of progressive causes as well representing nineteen trade unions.
He remained active, however, and was immediately appointed a member of the Royal Commission into Aboriginal Deaths in Custody, succeeding to the role of Commissioner in 1989 following Jim Muirhead's resignation.