Bruce was the second son of Sir william Waller Bruce, 10th Baronet (1856–1912) and his wife Angelica Lady Bruce (died 1917), daughter of General George Selby, Royal Artillery. Bruce was born in Ensenada, Baja California, Mexico, while his parents were on holiday there. His older brother was the author and adventurer Sir Michael Bruce. He was educated at the Grange, Stevenage and at Abingdon School, Oxfordshire. He served in France from 1914 as a lieutenant in the 10th Service Battalion of the Somerset Light Infantry and the Honourable Artillery Company, but was severely wounded at Cambrai the following year, with eleven bullets in his left leg, and spent most of the remainder of the war in a wheelchair.