He returned to military service prior to World War II, being commissioned as a pilot officer in the Royal Air Force Volunteer Reserve on 25 April 1939. On 17 June 1940, following Marshal Philippe Pétain's declaration of his intention to seek an armistice, Air-Vice Marshal Arthur Barratt, Air Officer Commanding-in-Chief British Air Forces in France, sent Grinnell-Milne on a mission to deliver letters to the leaders of the French Air Force, stating Britain's determination to continue the fight, and encouraging them to do the same in North Africa. Grinnell-Milne was also appointed the British liaison officer to the Commander-in-Chief of the French Air Force Joseph Vuillemin, and equipped with portable radio transmitter, call-signs and codes. Grinnell-Milne drove to Bordeaux, then to GHQ French Air Forces at Agen. There Vuillemin regretfully informed him that he had begun the process of transferring men and aircraft to North Africa, but had now received orders to halt all movements. Grinnell-Milne found most of the senior staff officers were either resigned to their fate, obstructive, or openly hostile, though he noted that many of the younger officers and men were eager to leave for England. He left France late on 18 June aboard . Once back in London he was appointed liaison officer to Free French General Charles de Gaulle, remaining with him for the next four months, and taking part in the battles of Dakar and Gabon. He was promoted to flying officer on 3 August 1941. Grinnell-Milne was invalided out of the RAF (technically he resigned his commission), retaining his rank of flying officer on 20 July 1944. Thereafter he rejoined the BBC, remaining there until 1946.