 He served in France, Salonica and then Palestine

After World War I he was Chaplain of Port Said and then Archdeacon of Egypt before his elevation to the Episcopate.During World War 2,he was praised for his ministry to British troops in Cairo and awarded a CBE

With the coming of the Suez Crisis and the heightened tension between Britain and Egypt that it brought, Johnston found himself expelled from the country 'virtually with the garments he was wearing'.He hoped to resume his pastoral duties in Egypt as soon as he was given permission to re-enter, but this authority never came.