In 1917, FitzGeorge Hamilton was commissioned as a second lieutenant in the Grenadier Guards, the regiment in which his great-grandfather, the Duke of Cambridge, also served.

[1][2][3] FitzGeorge Hamilton was named for his maternal great-grandfather, Prince George, Duke of Cambridge (1819–1904), Commander-in-Chief of the Forces from 1856 to 1895; his paternal grandfather, Sir Edward Archibald Hamilton, 4th and 2nd Baronet (1843–1915); and his maternal great-uncle Colonel Sir Augustus FitzGeorge (1847–1933).

[13] FitzGeorge Hamilton obtained a commission as a Second Lieutenant in the Grenadier Guards, an infantry regiment of the British Army, on 1 May 1917.

[5][17] On 18 May 1918, FitzGeorge Hamilton's battalion suffered an aerial bombing raid by enemy aircraft in Warlincourt-lès-Pas, France.

[10][17] With the permission of King George V, a memorial service was held for FitzGeorge Hamilton on 18 June 1918, at the Chapel Royal, St James's Palace, where he had been baptized 19 years prior.

[21] Memorials were also erected in his honour within the "Outer G3" section of the Winchester College War Cloister, and at St Mary's Church in Iping, near Midhurst.

[5] Following the war, FitzGeorge Hamilton's mother Olga gave birth to a daughter, Jane (4 June 1919 – 20 September 2014), with her second husband Robert Charlton Lane.

[22] His mother died in 1929, and in her will, she bequeathed a gold cup presented to FitzGeorge Hamilton by the Duke of Cambridge to the officers' mess of the 1st Battalion, Grenadier Guards.

[5][23] His mother also left £1,000 to Winchester College for the establishment of the George FitzGeorge Hamilton Fund to assist in the education of the children of Wykehamists who had died in the First World War.

[25] He married his third wife Lilian Maud Austen in 1927, and she subsequently converted to Islam and changed her name to Lady Miriam Hamilton.