[5] Grinnell-Milne was commissioned as a second lieutenant (on probation) in the 5th Battalion, Rifle Brigade (Prince Consort's Own) on 13 December 1913 at the age of 17.
[6] On the outbreak of World War I he was considered too young for front-line service, so in an effort to evade this restriction,[7] on 12 December 1914 he transferred to the 7th Battalion, Royal Fusiliers (City of London Regiment).
[14] Grinnell-Milne spent over two years as a prisoner of war before he finally escaped in April 1918,[15] making his way from Germany to The Netherlands and was briefly interned[16] before returning to England, where on 16 May he was presented to King George V at Buckingham Palace.
[1] On 21 October he was appointed a flight commander with the rank of acting-captain,[18][19] and celebrated by destroying a Fokker D.VII north of Bousies the same day.
[26] On 16 December 1920 he was awarded the Military Cross "in recognition of gallantry in escaping from captivity while [a] prisoner of war",[27] and on 1 January 1921 he was promoted to flight lieutenant.
[28] Grinnell-Milne then served as a test pilot at Royal Aircraft Establishment at Farnborough,[5] and in June 1922 took part in the third "RAF Aerial Pageant" at Hendon, where he and Flight Lieutenant P. W. S. Bulman flew two S.E.
[5] 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.
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.
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.
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.
This officer has shown exceptional gallantry and disregard of danger on numerous occasions, notably on 5th October, when he obtained a direct hit on a train with a bomb; he then attacked and destroyed in flames a balloon on the ground.
He married Blanca de Undurraga y Sandiford, daughter of Don Julio de Undurraga, which also ended in divorce, and he finally married Pauline Alice Margaret Lemieux, daughter of Dr. Louis-Joseph Lemieux (Canadian Member of Legislative Assembly for Gaspé), on 26 May 1932.