Seydlitz was born on 3 February 1721, in Kalkar in the Duchy of Cleves, where his father Daniel Florian Seydlitz, a major of Prussian cavalry, Curassier regiment Markgraf Friedrich Wilhelm of Brandenburg-Schwedt No. 5, was stationed.
By Seydlitz's seventh year, he could ride a horse well, raced with older boys, and he was, by most accounts, a wild and high spirited child.