Kazimierz Leski was born in Warsaw on 21 June 1912. His father, Major Juliusz Leski, had been an engineer and a pioneer of Poland's arms industry. following the Polish-Bolshevik War. However, he fell out of grace after the May 1926 Coup d'État, when he remained loyal to the government. Because of that, Kazimierz had to work as a railway worker to pay for his studies at the Wawelberg and Rotwand College in Warsaw. He also got a simple job in the foundry of the Pocisk munitions works. To study professional books, he learned English, Russian and German—abilities that proved invaluable later. Early on, he also learned French.