Dumanoir joined the Navy in 1787 as an élève de port and served in America until 1790. He was then promoted to ensign and served on the frigates Pomone and Néréide, cruising off Africa. He then embarked on the fluyt Dromadaire, bound for Cayenne. Promoted to lieutenant in 1790, he was appointed to the staff of Admiral Martin. He served on Sans-Culotte. Promoted to captain, Dumanoir took command of the Berwick in Richery's squadron, raiding commerce in the Mediterranean and in Newfoundland. In 1796, Dumanoir commanded a division under Admiral Bouvet during the Expédition d'Irlande, with his flag on Révolution. Upon his return to France, he encountered the Scévola, badly damaged in a storm, and rescued her crew. In 1798, Dumanoir took part in the preparation for the invasion of Egypt, and was tasked with supervising the harbour of Alexandria afterwards. The next year, he commanded the frigate Carrère, ferrying Lannes, Murat, Marmont and Parceval-Grandmaison to France, and sailing with the Muiron which carried ferrying Bonaparte, Gantheaume, Berthier, Andréossi, Monge, Berthollet, Denon, Lavalette et Bourienne. Dumanoir was promoted to contre-amiral on 21 November 1799, commanding a division of the squadron of Brest. During the Battle of Algeciras Bay, he was tasked with commissioning activities in Cadix, and after the battle, he was reprimanded for failing to reinforce Linois. In 1805, he was in command of a division in the French fleet commanded by Admiral Pierre-Charles Villeneuve. He took part in the Battle of Trafalgar on 21 October 1805, flying his flag on the Formidable and commanding the six-ship vanguard of the French fleet; cut off from most of the fighting, he gave the signal for his ships to flee the battle, and four of them did ; Dumanoir was chased down and captured by a squadron under Richard Strachan at the Battle of Cape Ortegal on 3 November and reached England in disgrace. He later was given commands in Danzig in 1811 and in Marseilles in 1815. He was promoted to vice-admiral on 27 January1819.
