Von Schmettau was born in Berlin. His mother, Marie de la Fontaine, belonged to a Huguenot family. His father died in 1707, as Royal Prussian secretary to the ambassador in London. In 1699, he joined the military, and served under numerous flags during his career. He started as a Danish Cuirassier, under command of his uncle William. In 1703, he joined the Margrave of Anspach Dragoons, as a lieutenant, where another uncle, Gottlieb Schmettau, was chief. Schmettau experienced his baptism of fire at the Battle of Blenheim. A little later he became captain and company commander, and was promoted to Major in 1707, and lieutenant colonel in 1708. He was Adjutant General of the Prince of Hesse, at the Battle of Malplaquet.