Marcognet was born on 14 November 1765 in Croix-Chapeau in Saintonge province, which is now the Charente-Maritime department. His parents were Louis-Nicolas, Count of Marcognet and Suzanne-Emilie Pintault, the count's first wife Suzanne-Louise Guicheneux having died on 30 March 1764. His great-grandfather, the Marquis de Montblin (died 1717) had been governor of La Rochelle, Saintonge, and Aunis. Marcognet entered the "Bourbonnais" Infantry Regiment as a cadet on 13 March 1781 and became a sous lieutenant in July. He served in the American Revolutionary War under Jean-Baptiste Donatien de Vimeur, comte de Rochambeau from 1781 to 1783. Promoted to lieutenant in 1787 and captain in 1792, he was assigned to the Army of the Rhine at the start of the War of the First Coalition. Marcognet was wounded in the right thigh on 14 September 1793 in fighting along the Lauter River. This was one of the skirmishes that preceded the First Battle of Wissembourg. In November he earned a name for himself in action near Saverne. He was wounded again at the Second Battle of Wissembourg when the French retook the Lines of Wissembourg.