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. On the outbreak of World War I he was considered too young for front-line service, so in an effort to evade this restriction, on 12 December 1914 he transferred to the 7th Battalion,
Royal Fusiliers (City of London Regiment). However, he was soon seconded to the
Royal Flying Corps for flying training, and on 17 August 1915 was granted Royal Aero Club Aviators' Certificate No. 1609 following a flight in a
Maurice Farman biplane at the Military Flying School at
Shoreham. He was confirmed in his rank of second lieutenant on 7 September, and two days later was appointed a flying officer. Grinnell-Milne was promoted to lieutenant on 29 September 1915, and posted to
No. 16 Squadron RFC, where on 28 November, flying a
BE.2c, he shot down an
Albatros C.I over
Sequedin. Only days later, on 1 December, he was forced down behind the German lines and captured. eventually being reported a prisoner of war in early January 1916. Grinnell-Milne spent over two years as a prisoner of war before he finally escaped in April 1918, making his way from Germany to The Netherlands and was briefly interned before returning to England, where on 16 May he was presented to King
George V at Buckingham Palace. Grinnell-Milne eventually returned to aerial combat with
No. 56 Squadron RAF, flying the
S.E.5a. and celebrated by destroying a
Fokker D.VII north of
Bousies the same day. He destroyed and drove down two more D.VII's over the
Mormal Woods on 29 October, and destroyed his fifth and final aircraft, another D.VII, on 3 November north-east of
Valenciennes. flying his red-painted S.E.5a, named
Schweinhund, for the last time on 23 January 1919. In February 1919 his award of the Distinguished Flying Cross was
gazetted. ==Between the wars==