Born in Caithness, Horne was educated at the Royal Military Academy, Woolwich. He served with the Royal Horse Artillery in India and in the South African War of 1899 and later in Ireland. On the outbreak of the First World War, Horne was appointed Brigadier-General and commanded the artillery of the First Army under Douglas Haig. In the retreat through the Marne, the Aisne and at Ypres, Horne was placed in charge of the rear-guard and did notable service. In 1915, Kitchener chose Horne to go with him to the Dardanelles and thereafter gave him the task of devising a defence scheme to cover the Suez Canal. Later Horne returned to France and took command of the First Army. For his war services, Horne received the title of Baron Horne.