Born the son of Major the Hon. Hugh Dawnay, son of the 8th Viscount Downe and Lady Susan de la Poer Beresford, daughter of the 5th Marquess of Waterford and educated at Eton and the Royal Military College, Sandhurst, Dawnay was commissioned into the Rifle Brigade in 1924 and then transferred to the 10th Royal Hussars later that year. He was part of the British polo team which won the silver medal in the 1936 Summer Olympics: he played both matches in the tournament, the first against Mexico and the final against Argentina.