Kania joined the Polish Workers' Party in April 1945 when the Germans were driven out by the Red Army and Polish Communists began to take control of the country. He partook in campaigns organised by the party, such as the Volunteer Reserve of the Citizens' Militia and the national referendum in 1946. He became deputy representative of the constituency for Jasielski. In February 1947 he was elected chairman of the ZWM board in Jaslo. Then in December he was appointed head of the Rural Youth Department in Rzeszow. In 1948, at the age of 21 he was elected as delegate to the unification congress of the Polish United Workers Party. After Edward Gierek was forced to resign as General Secretary amidst much social and economic unrest, Kania was elected his successor on 6 September 1980. He admitted that the party had made many economic mistakes, and advocated working with Catholic and trade unionist opposition groups. He met with Solidarity Union leader Lech Wałęsa, and other critics of the party.
After a KGB bug caught Kania criticizing the Kremlin, the Soviets forced him to step down in 1980. He was succeeded by Prime Minister Gen. Wojciech Jaruzelski. In 2012, he was tried for his role in the martial law crackdown in 1981, but acquitted.