After serving as page at the court of the margrave Frederick William of Brandenburg-Schwedt, Seydlitz entered the Prussian cuirassier regiment in 1740 and fought in the War of the Austrian Succession. In 1753 he was entrusted by Frederick II of Prussia with the task of imparting to poor regiments Prussian standards of perfection. Seydlitz performed this task admirably, but it was the Seven Years' War, fought between Austria and Prussia for supremacy in Germany, that brought out his true greatness. Seydlitz distinguished himself at Prague (May 1757), and his brilliant rearguard action after the Prussian defeat at Kolin (June 1757) earned him the rank of major general. He also commanded the cavalry at the battles of Rossbach (1757), Zorndorf (1758), Hochkirch (1758), Kunersdorf (1759), and Freiberg (1762).

After the Treaty of Hubertusburg (1763), Frederick promoted Seydlitz to general of cavalry and appointed him inspector general of Silesian cavalry. Toward the end of his life, Seydlitz, like many of his fellow commanders, fell out of favour with Frederick but was later reconciled with him.