Born in Molsheim (Bas-Rhin), François Joseph Westermann entered a cavalry regiment at an early age but soon left the service and went to Paris. He was an enthusiastic supporter of the Revolution, and in 1790 became greffier of the municipality of Haguenau. After a short imprisonment on a charge of inciting riots in Haguenau, he returned to Paris, where he joined Georges Danton and played an important part in the attack on the Tuileries on 10 August 1792.