 Millar entered the Diplomatic Service in 1923, and served in various capacities at the British embassies in Berlin, Paris and Cairo and at the Foreign Office.From 1934 to 1938 he was Assistant Private Secretary to the Foreign Secretary (Sir John Simon, Sir Samuel Hoare and Anthony Eden respectively).During the Second World War he served chiefly at the British embassy in Washington, where he was also Minister Plenipotentiary from 1948 to 1950.