He was educated at Cheltenham College and the University of Birmingham, before studying law at Middle Temple and medicine at Westminster Hospital.

He worked briefly for the Local Government Board in 1906 before joining the Colonial Service the following year.

Between 1907 and 1917, he worked in various administrative roles in Fiji, before being seconded for service in the First World War and attached to the War Office as a Royal Army Medical Corps officer.

St. Johnston was then colonial secretary in the Leeward Islands from 1920 to 1925, and was posted to Dominica, Antigua and Saint Kitts and Nevis between 1925 and 1929.

[4] During the Second World War he returned to military duties and joined the Ministry of Supply in 1942.