Crookshank was commissioned a second lieutenant in the Royal Engineers on 23 July 1887, promoted to lieutenant on 23 July 1890, and to captain on 1 April 1898. He served in the Second Boer War, and was slightly wounded in the battle of Paardeberg (February 1900). He was then attached to the 7th Infantry division of the South Africa Field Force. In 1932 he was assigned as the King's Body Guard.