He was born in Peterskirchen/Oberbayern into a Bavarian family. His father was the teacher Johann Anton Rauch (c 1686–1745). He began his career in 1756 as a page at the court of the Principality of Brunswick-Wolfenbüttel and in 1761 became an engineer in the Brunswick Army. Under Charles William Ferdinand, Duke of Brunswick-Wolfenbüttel he took part in the closing phases of the Seven Years' War, namely in the 1761-62 sieges of Kassel (in which he was wounded), Meppen and Ziegenhain. In 1764 he was made a conductor in the Brunswick Engineer Corps, rising to lieutenant in 1766 and captain in 1772.
