Josias joined the Habsburg military as Colonel in 1759, participated in the Seven Years' War, and rose to the rank of lieutenant field marshal by 1773.

In the Russo-Turkish-Austrian war of 1788, he commanded an army corps under Freiherr von Laudon, occupying Moldavia, capturing Khotyn in Bessarabia and sharing in Aleksandr Suvorov's victory in the Battle of Focșani (1 August 1789).

Having completely beaten the main Ottoman army under Grand Vizier Koca Yusuf Pasha in the Battle of Rymnik, he captured the greater part of Wallachia, including Bucharest, being welcomed by the population after the flight of Prince Nicholas Mavrogenes (see History of Bucharest), and soon after becoming a field marshal.

Frederick was ennobled by the Austrian Emperor on 25 August 1808 and on 17 February 1853 the Duke Ernst II of Saxe-Coburg-Gotha created him Freiherr von Rohmann, named after the place of his birth.

Angered by this, and openly criticizing the policies of the Baron Thugut, Josias resigned as Field Marshal (the Count of Clerfayt assumed command in his place) and retired to Coburg, where he later died.