Born in Mannheim on 11 December 1743, Bernhard Erasmus von Deroy's parents were General Matthias Bertram de Roy and Elizabeth Christine of Hofstatt. He was appointed ensign in the "Count Palatine Karl August" Infantry Regiment at a very young age and fought in the Battle of Hastenbeck on 27 June 1757. He became a lieutenant on 14 February 1761 and captain of an infantry company on 5 September 1763. His next promotion occurred on 30 October 1775 when he became a major in the "Duke of Zweibrücken" Infantry Regiment. He was elevated in rank to Oberstleutnant while belonging to the "Rodenhausen" Infantry Regiment on 8 June 1784. On 21 December 1787 he returned to the "Duke of Zweibrücken" as Oberst and commanding officer. On 3 November 1792, Deroy was promoted to "General-Feldwachtmeister" and later appointed governor of Mannheim. In this position he became responsible for strengthening the fortifications in anticipation of a French attack.