After being commissioned into the Grenadier Guards in 1885, he served in the Second Boer War as a company commander, then served with distinction during the First World War as a brigade, divisional, corps, and army commander, and later advised the British government on the implementation of the Geddes report, which advocated a large reduction in defence expenditure; he presided over a major reduction in the size of the British Army.

Born into an aristocratic family of Anglo-Irish descent, he was the son of the 9th Earl of Cavan and Mary Sneade Lambart (née Olive).

[4] He gained the courtesy title of Viscount Kilcoursie in 1887 when his father succeeded to the Earldom and was appointed aide-de-camp to Frederick Stanley, the Governor General of Canada, in 1891.

He saw action as a company commander in the Battle of Biddulphsberg in May 1900,[10] and, having succeeded to his father's titles on 14 July 1900,[11] took part in operations against the Boers in 1901 and for which he was later mentioned in despatches.

[12] Following the end of the war in June 1902, which prompted him to write in his diary that it was "not far removed from the happiest day of my life",[11] he left Cape Town on the SS Sicilia and returned to Southampton in late July.

[15][16] He was appointed a Member of the Royal Victorian Order Fourth Class on 29 June 1910,[17] which was awarded personally to him by George V.[18] He was promoted to colonel on 4 October 1911,[19] After four years as CO of his battalion, he was placed on the half-pay list.

[36][37] The following January 1916, Cavan, "his star in the ascendant",[38] was promoted to temporary lieutenant-general[39][15] and was placed at the head of XIV Corps and took part in the Battle of the Somme that summer.

[15] Advanced to Knight Commander of the Order of the Bath on 1 January 1918,[45] Cavan was appointed Commander-in-Chief (C-in-C) of the British Forces in Italy on 10 March 1918, after his predecessor, General Herbert Plumer, had been recalled to the Western Front after the Germans had launched their Spring offensive.

[46][15] After reverses on the Western Front in March and April 1918, Prime Minister Lloyd George and the War Cabinet had been keen to remove Field Marshal Sir Douglas Haig as C-in-C of the BEF, but had been unable to think of a suitable successor.

[66] Earl Cavan made a famous speech at the 'Royal Academy Banquet' to his equals in government and fellow peers and royalty.

[71] In May 1927, he accompanied the Duke and Duchess of York to Australia to open the Provisional Parliament House at Canberra, for which he was appointed a Knight Grand Cross of the Civil Division of the Order of the British Empire on 8 July 1927.