In suggesting de Fleurieu's promotion to ensign, on 23 March 1762, the Minister wrote to the king: he combines the wisest of conducts and greatest of dedications with extraordinary knowledge and the most favourable disposition to become a distinguished officer.

He took part in a one-year sea campaign to test Berthoud's first marine chronometer, in an attempt to beat Britain in the race to find a reliable way to calculate longitude.

Knowing the actual local time at each present location by astronomy, they could easily determine the ship's exact position and longitude on a chart.

He was presented to the king and named capitaine de vaisseau on 5 December 1776 and soon afterwards director of ports and arsenals in January 1777, a post heading the fleet's organization of matériel, works and movements created specially for him by Louis XVI and one he held for 15 years.

In it he directed nearly all planning for naval operations against the British as part of the American Revolutionary War, as well as all the French voyages of discovery such as that of La Pérouse.

He remained imprisoned with his wife in the Madelonnettes for 14 months; they were finally released to find their homes, furniture, lands and resources dispersed and destroyed.

On 30 September 1800, as Minister Plenipotentiary, he signed a treaty of friendship and commerce between France and the United States at Morfontaine, alongside Joseph Bonaparte.

[citation needed] At age 54, he married Aglaé-Françoise Deslacs d'Arcambal; they had one son who died young and two daughters, including Caroline (Madame de Saint-Ouen, from whom his descendants trace their lineage).