In September, after the beginning of the American Revolutionary War, he served on the Nymphe, taking part in two battles against British privateers near Bordeaux and Belle-Isle.

He served successively aboard Pégase on February 1782 and the Protecteur in March before taking command of the fluyt Fidèle in Terre Neuve from May 1783 to April 1784.

He accompanied the squadron to the West Indies and took part in the capture of the Diamond Rock from the British, 6 nautical miles (11 km) away from Martinique.

Back to Europe, he took part in the Battle of Cape Finisterre, on 22 July, preventing one Spanish warship from being captured by the British.

On 23 October, Cosmao, as one of the most senior officers present, jointly took command of the squadron with Spanish Commodore Enrique MacDonell,[1] and set back to sea with five ships: Pluton, Héros, Neptune, San Francisco de Asis, and Rayo.

He took command of a division of the French Mediterranean Fleet, under Ganteaume from 1807. he took part in operations to resupply Corfu and to the landing in Sicily.

Cosmao set his flag on Robuste and took command of a squadron comprising Donawerth, Génois, Borée and Lion, as well as the frigates Pauline and Pénélope, and a dozen of transports.

He escaped the forces of Admiral Pellew and successfully returned to Toulon on the 10th with the Scorpion without losing a ship, though the Romulus, at the rear, did suffer some damage.

During the First Restoration, in April 1814, Cosmao-Kerjulien received command of the Mediterranean Fleet, and was appointed a Knight of the Order of Saint Louis.

In 1815, upon the Emperor's brief return to power, he rallied to Napoleon, who made him préfet maritime of Brest in March, and a Peer of France in June, right before the Battle of Waterloo.