[2] Johnston was born at Maryport, Cumberland, the son of a Baptist clergyman, and was educated at Banbury Grammar School and The Queen's College, Oxford.
In 1956–57, Johnston was head of the Far Eastern Department of the Colonial Office, concerned with delivering independence to Malaya and the future of Singapore.
He then transferred to the Commonwealth Relations Office (CRO), where he was head of the Defence and Western Department before being appointed deputy high commissioner in South Africa in 1959.
Johnston had to deal with what he described as "hardcore racialists"[3] in the Rhodesian Front government (under Ian Smith), as well as with the African nationalists leaders Joshua Nkomo and the Rev Ndabaningi Sithole.
So tense was the atmosphere in the Rhodesian capital, Salisbury, that Johnston found it impossible to establish relaxed friendships.