Viomesnil entered the French army at age 13 as a lieutenant of the regiment of Limousin. He served in the War of the Austrian Succession where he took part in the Battle of Lauffeld and the capture of Bergen op Zoom. During the Seven Years' War he served as an aide de camp to general François de Chevert (1757–1758). Viomesnil was made a knight of the Order of Saint Louis in 1760. He was promoted to colonel in 1761. In 1768 he commanded an infantry brigade during the annexation of Corsica. Upon his return to France in 1769 he was made colonel of the Légion de Lorraine en 1770.