L'Estocq was born in Celle, Electorate of Hanover, the son of a Huguenot Prussian officer. In 1757 he became a cadet officer of the Gensd'armes regiment of Berlin. During the Seven Years' War, he participated in the battles of Zorndorf, Kunersdorf, and Torgau. After a battle near Langensalza, he received the Pour le Mérite.