He was born in 1856, the second child and eldest son of George Baillie-Hamilton-Arden, 11th Earl of Haddington. After an education at Eton and Trinity College, Cambridge, he was commissioned in the Royal Horse Guards on 11th September 1880. He served with distinction in the 1882 Anglo-Egyptian War, the Nile Expedition of 1884, and the Hazara campaign of 1888. In 1889 he was appointed aide-de-camp to the Viceroy of India, and from 1899 to 1903 held the command of the Royal Horse Guards. He retired from the army in 1907, but remained in the Territorial Force as commanding officer of the Lothian and Border Horse.
He was appointed a temporary Brigadier-General in December 1915, and given command of 41st Brigade in 14th (Light) Division. He remained in command until April 1916, returning home to command 11th Mounted Brigade.
In 1892 he had married Katharine Salting, with whom he had two sons and a daughter. He died a few months before his father, and so did not succeed to the title of the Earl of Haddington; instead, it passed to his eldest son, George Baillie-Hamilton.