Johnstone was awarded the Military Cross (MC) in August 1943, for his services at Enfidaville, Tunisia, in April/May 1943.

On 22 October 1954 he was appointed a Member of the Order of the British Empire (MBE) in recognition of distinguished services in Malaya during the period 1 January to 30 June.

[4] From 1955 to 1957 he was officer in charge of the medical division at the Cambridge Military Hospital, Aldershot Garrison, Hampshire.

From 1963 to 1965 he was commanding officer of British Military Hospital (BMH), Iserlohn, Germany, and from 1965 to 1967 he was consultant physician to HQ, Far East Land Forces (FARELF).

[5] Following his retirement from the Army in 1969 he served as assistant director (overseas) of the British Postgraduate Medical Federation from 1970 to 1976.