Lonsdale was born in Den Helder, Batavian Republic, during Britain's failed campaign to restore the deposed Prince William of Orange. His father, Lieutenant James Lonsdale, had been accompanied by his wife Jane ("née" Faunce). William at age 20 joined his father's old regiment, the King's Own (4th) Regiment of the Foot, as an ensign on 8 July 1819. He was soon joined by his younger brother, . William and served with their Regiment in the West Indies and on 4 March 1824 William was promoted Lieutenant and appointed Adjutant. He returned to England and was posted to Portugal. By 1830 he was back in England and by 20 March 1831 his Regiment was posted to the Colony of New South Wales to relieve the 39th Regiment. He was to become a colonial administrator as well as his profession as a soldier.