At Oxford, Ruggles-Brise obtained a Second Class in Classical Moderations, "long regarded as one of the hardest examinations in the world", and his cricket Blue in 1883.

[3][4][7] After Oxford, Ruggles-Brise entered the Royal Military College, Sandhurst, passing out in 1885,[8] and was commissioned as a lieutenant in the Grenadier Guards in May 1885.

[3][24][10][11] In October that year he and his brigadier, Sir Henry Colville, were transferred to Cape Colony as part of the troop build-up for the Second Boer War.

[33] Ruggles-Brise remained in South Africa until the end of the year, when he was re-appointed as brigade major[34] in Home District (London).

This had been turned down on grounds of cost, so the decision had been made to train the infantry in rapid-fire musketry to make up for the lack of automatic weapons.

During Ruggles-Brises's command the school played a crucial role in training the instructors who in turn taught the British Regular Army to shoot so effectively that in the early part of the First World War German reports repeatedly credited them with possessing large numbers of machine guns.

[47] Soon after the outbreak of the First World War, Ruggles-Brise was promoted to temporary brigadier general (15 September)[10][48][49] to command a brigade composed of the last three infantry battalions of the Regular Army left in Britain after the British Expeditionary Force (BEF) went to France.

In the event all it could do was help to cover the Belgian retreat and then take up defensive positions at Ypres where they were joined by the rest of the BEF after the race to the Sea.

Thereafter the 20th Brigade was engaged in heavy fighting at Langemarck and Gheluvelt during the First Battle of Ypres[52][57][58] Like several other senior officers who got out among their units to exercise personal command during this confused fighting, Ruggles-Brise was wounded, having "sustained dreadful wounds to both arms and his shoulder blade and was stretchered back half dead, leaving Major Cator in command [of the 20th Brigade].

[65] The 40th Division under Ruggles-Brise embarked for France in early June and took its place on the Western Front to join in the continuous trench warfare.

[10][72] His next posting was as military secretary at the general headquarters (GHQ) of the British Expeditionary Force under the BEF's commander-in-chief (C-in-C), Field Marshal Sir Douglas Haig.

[10][73] After harsh criticism of GHQ's performance during the 1917 fighting, several of Haig's senior staff had been replaced by new men like Ruggles-Brise brought in.

He then worked in the Military Secretary's Department in England until 3 September 1919,[10][80] and retired from the army, after thirty-five years of service, on 10 March 1920.

[3][4] As CO of the 3rd Battalion, Grenadier Guards, he was made a Member of the Royal Victorian Order (MVO) – a personal award by the King.

[88] In 1895 Ruggles-Brise married music expert Lady Dorothea Stewart Murray (1866–1937), elder daughter of the 7th Duke of Atholl.