Of noble Burgundian descent, he was a student at the Brienne military school, then was a graduate of the Paris military school. Nansouty began his military career in 1785, as a sub-lieutenant in the regiment "Bourgogne-Infanterie", where his father had served during the wars of Louis XV. A cavalry officer at the outbreak of the French Revolutionary Wars in 1792, Nansouty was commissioned as an "aide-de-camp" to Marshal Nicolas Luckner. During the First Coalition, he saw service as a lieutenant-colonel and squadron commander in the 9th (heavy) Cavalry Regiment, campaigning with the French armies on the Rhine and in Germany. Promoted to Colonel in 1793 and given the command of the 9th Cavalry, he was noted for several well-led cavalry actions. Finally made a Brigadier General in 1799, after he had refused the promotion several times in the past, Nansouty fought the next year under General Jean Victor Moreau in southern Germany, in a decisive campaign of the Second Coalition.