The fourth son of Ferdinand Albert II, Duke of Brunswick-Lüneburg, Ferdinand joined the Prussian army as a colonel in 1740. He was present in the battles of Mollwitz and Chotusitz. After Margrave Wilhelm of Brandenburg-Schwedt was killed at Prague in 1744, Ferdinand received command of Frederick the Great's Leibgarde battalion, and at the Battle of Soor (1745) he distinguished himself greatly, especially in the assault of a steep hill, that incidentally was defended by his older brother duke Louis Ernest of Brunswick-Lüneburg. He took part in the Second Silesian War before leading part of the invasion of Saxony and Bohemia in 1756 during the Seven Years' War. He participated in the Battle of Rossbach, and then became commander of the allied Hanoverian army.