Horne was born on 19 Feb 1861 in the parish of Wick in Caithness, Scotland, the third son of Major James Horne and Constance Mary Shewell. He was first educated at Harrow, receiving an artillery commission from the Royal Military Academy, Woolwich in May 1880, when he was appointed a Lieutenant in the Royal Field Artillery. Promotion to Captain followed on 17 August 1883, and to Major on 23 February 1898.
From 1899 to 1902 Horne fought with the cavalry in the Second Boer War in South Africa under Sir John French. He received the brevet promotion to Lieutenant colonel on 29 November 1900, and in the latter stages of the war served as a remount officer and was mentioned in dispatches. In 1905 he received the substantive promotion to lieutenant-colonel and served with the Royal Horse Artillery under Sir Douglas Haig. His military career was unremarkable until 1912 when he was promoted to brigadier and appointed Inspector of Artillery.
War broke out two years later and Horne was appointed to command a force of artillery under General Haig, who commanded I Corps. At the Battle of Mons, Horne distinguished himself with a rearguard action that allowed Haig's I Corps to retreat almost effortlessly; admittedly the German Army made few attacks toward Haig's forces, as they were occupied by Sir Horace Smith-Dorrien's costly defensive action.
Horne fought with distinction in the British Expeditionary Force's actions throughout 1914; in October of that year, he was promoted to major general and created a Companion of the Order of the Bath. A few months later, he was given command of the 2nd Division. In May 1915, Horne's division participated in the first British night attack of the war, distinguishing itself at the Battle of Festubert, France; the attack faltered, partly because the artillery ran out of ammunition. The media launched vicious attacks on the Secretary of State for War, Lord Kitchener; the blame was eventually laid on General French who was sacked at the year's end. Significantly, the artillery were reorganised after this fiasco at General Horne's suggestion.
In November 1915, Horne accompanied Lord Kitchener to the Dardanelles, where they organised and executed the evacuation of Gallipoli. For several months, Horne was placed in charge of the Suez Canal defences (and given command of the XV Corps).
March 1916 saw him back on the Western Front. He joined the Fourth Army which was preparing for an attack on the Somme. On 1 July 1916, General Horne's XV Corps participated in the costliest battle of the First World War. In the pre-battle plans, Horne advocated and became an architect of the "creeping barrage" which was used for the rest of the war. The Battle of the Somme, which raged on for four months, ended with over a million casualties on both sides—more than 57,000 casualties occurred on the first day. 13 British divisions participated in the attack; General Horne's XV Corps consisted of the 21st and 7th Divisions. His divisions focused their attack on two villages, Fricourt and Mametz; they captured both on the first day for about 8,000 casualties.
In September 1916, Horne was created a Knight Commander of the Order of the Bath and, after the successful capture of Flers, he was promoted to temporary general and succeeded Sir Charles Monro (after a brief hiatus between 7 August and 29 September when the command was held by Haig's first choice Richard Haking, who was then blocked from receiving the promotion) as commander of the First Army. On 1 January 1917, he was promoted to the substantive rank of Lieutenant-General "for distinguished service in the field". His first trial occurred in April 1917, when his troops were sent on a diversionary attack on the fearsome Vimy Ridge, which rose hundreds of feet over the surrounding landscape. French Army commander Robert Nivelle was critical of Horne's plan; Nivelle was the one found incompetent and, after one month of relative failure (and a mutiny), Nivelle was sacked and replaced with Philippe Pétain.
The attack on Vimy Ridge was spearheaded by the First Army's "shock troops" (the Canadian Corps). The ensuing Battle of Vimy Ridge, the first of a series of actions known as the Battle of Arras was successful: supported by Horne's 1,000-odd artillery pieces, the Canadian forces took the ridge in four days, with approximately 10,000 casualties (against 20,000 German casualties). The capture of Vimy Ridge would prove essential to the British Army: it served as the backbone of the British defence from March 1918 onward.
Nivelle's failure and sacking lengthened the actions around Arras. With success imminent, General Haig began siphoning troops northward, where many would participate in the Battles of Messines and Passchendaele. The First Army served mainly as a diversion and a placeholder until April 1918.
In April, the Germans embarked on the Spring Offensive which was similar to the Allied Somme Offensive two years previous. At first, the attack was successful. On Horne's front, nine German divisions attacked his weak left flank which was manned by two exhausted Portuguese divisions. The Germans advanced six miles to the banks of the River Lawe, where they were repulsed by the 55th and 51st Divisions.
After this final German offensive, the British took the initiative permanently. General Haig's forces embarked on the Hundred Days Offensive, which ended the war; General Horne's troops distinguished themselves in the lengthy offensive.
At the end of the war, Horne was created a Knight Commander of the Order of St Michael and St George and a Knight Grand Cross of the Order of the Bath. For his wartime services he received the thanks of Parliament and was raised to the peerage as Baron Horne, of Stirkoke in the County of Caithness. He was promoted to head of the Eastern Command in 1919 and retired from the army in 1923. He was appointed Master Gunner of St. James's Park, an honorary position he would hold until his death; he was also appointed Colonel of the Highland Light Infantry.
Lord Horne married Kate, daughter of George McCorquodale, in 1897. While shooting in his Stirkoke estate in August 1929, he suddenly died of unknown causes, aged 68. He was buried on his family plot. Although by a special remainder his title could be inherited by a male grandchild, his only child, daughter Kate (also known as 'Kitten'), also only had daughters so the title became extinct.
It was believed that he had not kept a diary and that his wife had destroyed all his letters after his death, although, in reality, his papers had been handed down to his granddaughters, who had kept them safe. The donation of his extensive papers, which include his diaries and letters, to the Imperial War Museum by the family has allowed his career to be re-evaluated.