In June 1791 Ernst Marschall von Bieberstein entered military service as a lieutenant in the district militia ("Kreiskontingent") under the command of Prince William of Nassau-Usingen. However, he was already destined, in the longer term, for a career in and after a year of military service he made the switch, taking a post as Court and Government Assessor. Despite his relative youth, in 1793 he travelled to The Hague on a diplomatic mission. However, he found himself arrested and detained by French troops. He was evidently soon released, since his political career serving the Duchy of Nassau took off. He was appointed a later in 1793, and in 1795 became a member of the Privy Council. Further promotion followed, and he emerged as one of the most influential members of the government. The pressing issue of the times was the aggressive advance across western Europe of the French Revolutionary Army. Given the overwhelming superiority in terms of resources and numbers of the French forces, Marschall von Bieberstein was pragmatic, counselling a conciliatory approach. As early as 1793, echoing the views of the men who later came to be known as the Prussian reformers in Berlin, Marschall von Bieberstein had shared his opinion that the best protection against the revolutionary tide lay in adopting a constitution, though it would be another twenty years before he would have the opportunity to on this. In order to diminish revolutionary tendencies he also warned against imposing excessive "feudal burdens" on the population. In 1797, in defiance of the wishes of , that Ernst Marschall von Bieberstein undertook a diplomatic mission to Paris as part of a mediatisation policy, apparently of his own.