[4][5] Fortescue was educated at Harrow School and, after graduating from the Royal Military College at Sandhurst, was commissioned as a second lieutenant into the Rifle Brigade (The Prince Consort's Own) on 22 January 1881.
[4][5][7][8][13] Fortescue returned from West Africa in 1899 to take up the post of private secretary to the Secretary of State for War, the Marquess of Lansdowne, but just before the outbreak of the Second Boer War he went to South Africa as a brigade major in the Natal Field Force under Sir George White.
[15][5][8] He left Port Natal on the SS Malta in late September 1902, together with other officers and men of the 2nd Battalion, Rifle Brigade who were transferred to Egypt.
On 14 March the Germans made a surprise attack on 80th Brigade in the late afternoon, firing two mines and capturing St Eloi village, the surrounding trenches, and an artificial heap of earth known as 'The Mound'.
There was severe hand-to-hand fighting in which the 2nd King's Shropshire Light Infantry and 4th Rifle Brigade distinguished themselves, but Fortescue was unable to make an immediate counter-attack because no reserves were on hand.
[5] In November 1916 he received command of 212th Brigade of the 71st Division, a new Home Defence formation composed of men who were unfit for overseas service.
[28] In 1906 Fortescue married Ethel Rosa, daughter of General Sir Charles Clarke, 3rd Baronet and widow of Captain Ernest Campbell.
They had two daughters:[4] Before the First World War, Fortescue had accompanied his historian brother to some of the old battlefields of Europe, and he did a considerable amount of research for the last volume of Sir John's History of the British Army, which appeared in 1930.