In 1783 he joined the army of a German state and became a lieutenant of dragoons within three years. In 1791 he joined the French army as an infantry captain. He fought in the French Revolutionary Wars, becoming an Adjutant General Chef de brigade (colonel) on 12 April 1794.
Napoleon Bonaparte named him a commander of the Légion d"Honneur on 14 June 1804.
Assigned to lead a brigade in the VI Corps, he fought at Haslach-Jungingen and Dürrenstein in the War of the Third Coalition. He earned advancement to general of division on 24 December 1805 but continued to lead a brigade.
The following year he fought at Halle, Waren-Nossentin, and Lübeck. In the last action he captured Carl Carlsson Mörner and 600 Swedes.
In 1807, he led his troops at Mohrungen and Braunsberg before being given command of a Hessian division.
He became a Baron of the Empire on 18 March 1809. In the War of the Fifth Coalition his division of French-allied Germans guarded Napoleon"s line of communications. His forces were mauled at Franzensfeste on 4 August 1809 while attempting to suppress the Tyrolean Rebellion.
In early 1810 he led a division in Catalonia.
In 1813 and 1814, he commanded the 2nd Division in Italy at Caldiero and the Mincio. King Louis XVIII of France appointed him a chevalier of the Order of Saint Louis.
He was buried at Barville, Vosges on 10 August 1824. ROUYER is one of the names inscribed under the Arc de Triomphe, on Column 1.