MacDonell's service career began in the 1930s. Shortly before the war he became a Squadron Leader and worked at the Air Ministry during the Phoney War. When hostilities commenced he became CO of No 64 Squadron, carrying out convoy support operations and eventually fighting in the Battle of Britain. Awarded a DFC, he was given command of a squadron at Leconfield to train urgently required pilots. Eventually he was shot down over the English Channel and rescued by a U-boat, this resulted in a lengthy period as a PoW in camps throughout enemy occupied Europe and Germany. During this period he was involved with the famous 'Wooden Horse' escape and was eventually freed by advancing Russian troops. Upon his return to the UK he was promoted Wing Commander and worked on the Cabinet Office staff before moving to Headquarters Flying Training Command. He was then appointed Chief Flying Instructor at Cranwell before successfully applying for the post of British Air Attaché in Moscow.