He was the eldest son of prominent émigré diplomat François-Emmanuel Guignard, comte de Saint-Priest (1735–1821), one of King Louis XVI of France's last ministers. Guillaume Emmanuel became a major-general in the Russian army under Czar Alexander I of Russia, and fought against the forces of Napoleon. Some weeks before the Battle of Leipzig, he and his cavalry finally defeated the troops of French brigade general François Basile Azemar in the Battle of Grossdrebnitz. Saint-Priest was defeated and mortally wounded during the 1814 Allied invasion of France in the battle of Reims and died two weeks later at Laon.