Siwicki was born in Łuck, Poland. In the course of his career he held a number of posts, including military attaché in China, commanding officer of the 2nd Polish Army during the invasion of Czechoslovakia in 1968, the Chief of General Staff of the Polish Army and a long-time Minister of Military Affairs in the governments of Wojciech Jaruzelski, Zbigniew Messner, Mieczysław Rakowski and Tadeusz Mazowiecki. As one of the people behind the imposition of the martial law in Poland in 1981, after Poland deposed the ruling communist regime in 1989 Siwicki was forced into retirement in July 1990.