When the French Revolution began in 1789, Jean Louis Debilly was a professor of mathematics in Paris. He joined the Parisian National Guard artillery in 1792, and served for six weeks as provisional commander of the artillery defending the coast at Brest. In June he entered the army, and was promoted to adjutant general. Debilly declined a proffered promotion to remain as General Kléber's chief of staff. He served for a brief time with Army of England, until the Directory abandoned the project. He was transferred to Jean-Baptiste Jourdan's Army of the Danube, where he filled several roles: as commander of cavalry, he supported Dominique Vandamme's detached flank maneuver on Stuttgart, and he was a commander of the left column of the III. Division of the Army at the first Battle of Stockach. He was badly wounded at the French defeat at the First Battle of Zurich in early June 1799, and was unable to ride a horse for several months. Massena promoted him to brigadier general, and assigned him to a staff position as chief artillery and engineers of the III. Division.