Cillart was born to the family of Madeleine de Bernard and of Joseph-François de Chabert, a Navy officer. He joined the Navy as a Garde-Marine in 1741, and took part in the Battle of Toulon at Cape Sicié on 22–23 February 1744, serving on the 50-gun "Diamant". In 1745, he served on "Trident" and "Espérance" in two missions to Martinique. In 1746, he was on "Castor" and took part in the capture of the British corvette "Albany" off Acadia. The year after, he was taken prisoner at the First Battle of Cape Finisterre on 14 May 1747. Chabert was promoted to Ensign in 1748.