Major William Knox-Leet was born in Dalkey, County Dublin on 3 November 1833. The youngest son of a Episcopalian rector, he graduated from Trinity College in Dublin, Ireland and became an ensign in the 13th Prince Albert's Light Infantry in July 1855. In 1858 he was sent to British India where he saw action in several engagements during the Indian Mutiny. From 1867 until 1872 he served as an arms instructor in England. In July 1872 he became Deputy Assistant Adjutant and Quartermaster-General in County Cork, Ireland.