At the outbreak of the Anglo-French War in 1778, Grenier was serving on the 64-gun "Sphinx". He took part in the Battle of Ushant, and was then given command of the frigate "Boudeuse" in the squadron under Estaing. On 22 January 1779, he captured the British corvette "Weazle". He took part in the Battle of Grenada and in the Siege of Savannah in September 1779. In 1780, he was on convoy escort duty. Grenier was promoted to Captain on 9 May 1781, and served on the "Bretagne". He then worked on supplies for Guichen's squadron.