du Rietz began his military career as a lieutenant in the Life Guards and was sent to the Finnish front in the Russo-Swedish War (1741–1743) with that regiment. After that war he rose slowly through the officer ranks, reaching major in the Södermanland Regiment, and then went to join the Pomeranian War in north Germany from 1758 to 1760. He won distinction in this war, capturing the Anclamen Feler fort, protecting the Swedish siege of Peenemünde and obtaining the advantageous capitulation of the Prussian forces at Demmin by Field Marshal Johann von Lehwaldt.