Fleurieu was born in Lyon. He joined the Navy on 31 October 1755 at Toulon as a Garde-marine, aged just 13 and a half. He subsequently took part in the campaigns of the Seven Years' War— which ended in 1763— participating in the battles of Mahon, Lagos, and Les Sablettes and rising to brigadier in the "gardes de la marine" company, and then "enseigne de vaisseau". In suggesting de Fleurieu's promotion to ensign, on 23 March 1762, the Minister wrote to the king: