When the French Revolution (1789–1799) broke out, Lambert supported it enthusiastically. On 18 May 1790 he was appointed adjutant in the national guard of Burgundy, and in 1792 he was named second in command. On 5 September 1792 he was elected lieutenant colonel and second in command of the battalion of grenadiers of the Côte-d'Or. He was attached to the Army of the North, which he joined soon after the Battle of Jemappes in November, and with whom he served in the Belgian campaign. His battalion was then assigned to the Army of the Alps, which freed Lyon from the royalists.