In 1789, Lefebvre was a sergeant in the Gardes Françaises. After its disbandment, he enlisted in the National Guard, where he received wounds protecting the Royal Family from an angry mob, after which he joined the revolution. Promoted to brigadier general in 1793, he took part in the Battle of Fleurus (24 June 1794). After General Louis Lazare Hoche's death he commanded the Army of Sambre-et-Meuse (September 1797). He then commanded the vanguard of the Army of the Danube under Jourdan in March 1799, although for the first week of the campaign he was incapacitated with ringworm and Dominique Vandamme replaced him temporarily. He was later injured at the Battle of Ostrach where the Advance Guard bore the brunt of the early fighting. In November 1799, Lefebvre commanded the Paris troops and agreed to support Napoleon Bonaparte in his "coup d'état". In 1800, Bonaparte appointed him senator.