In the Seven Years' War, his brilliant conduct at the churchyard of Leuthen and at Hochkirch won him promotion to major. In 1760, in the almost lost Battle of Torgau, Möllendorf was captured by the Austrians, but released in 1761, and was made major general later in the year. He received an Order of Merit for his exertions. At the Battle of Burkersdorf Möllendorf commanded a brigade that participated in the successful assault of the fortified Austrian positions.
Seventeen years later, as lieutenant-general, he won at Brix one of the few successes of the War of the Bavarian Succession (or Potato War).
In the years of peace Möllendorf occupied considerable posts, being made governor of Berlin in 1783. Promoted to general of the infantry in 1787, and general field marshal in 1793, that year he commanded the troops which put into effect the second partition of Poland. He commanded the Prussian army on the Rhine in 1794.
In the disastrous Battle of Jena-Auerstedt (1806) Möllendorf played a considerable part, though he did not actually command a corps. He was present with King Frederick William III at Auerstedt. He was wounded, and fell into the hands of the French, in the debacle which followed. After releasing him, Napoleon awarded him the cross of the Legion of Honor.