Lehwaldt was born in Legitten near Labiau in the Duchy of Prussia. His mother Marie Esther came from an old Prussian family, the Freiherrn von der Trenck. In 1699, Lehwaldt began his military service when he entered the battalion "Weiße Grenadier-Garde" (Nr. 18). He participated in the War of the Spanish Succession since 1702 and experienced his trial by fire in the September siege of Venlo. In spring 1704, his battalion fought in the Battle of Blenheim, and Lehwaldt was promoted to Fähnrich on 16 September. From 29 September – 6 October he participated in the siege of Hagenau.