The son of William B. and Janet Blane, Caldwell was born in Glasgow on 28 December 1880. By 1911, he was a boarder in the home of John Cuthbertson at Carlton Road, Wolverhampton. Caldwell was working as a hardware merchant. In 1912, he married Agnes Herbertson Shepherd Blane in Greenock. The couple had two sons – William Shepherd and Douglas H. – in Wolverhampton in 1917 and 1919.
During the First World Caldwell does not appear to have served in the Army. However, he did serve locally in Wolverhampton as a Special Constable, and survived the war.
In 1939, the couple and their son, William, were living at 11 Riley Crescent, Wolverhampton, and Caldwell was a lock manufacturer. Caldwell died in Wolverhampton in 1967.