Kádár was born in Fiume in poverty to a single mother. After living in the countryside for some years, Kádár and his mother moved to Budapest. He joined the Party of Communists in Hungary's youth organization, KIMSZ, and went on to become a prominent figure in the pre-1939 Communist party, eventually becoming First Secretary. As a leader, he would dissolve the party and reorganize it as the Peace Party, but the new party failed to win much popular support.

After World War II, with Soviet support, the Communist party took power in Hungary. Kádár rose through the party ranks, serving as Interior Minister from 1948 to 1950. In 1951, he was imprisoned by the government of Mátyás Rákosi but was released in 1954 by reformist Prime Minister Imre Nagy. On 25 October 1956, during the Hungarian Revolution, Kádár replaced Ernő Gerő as General Secretary of the Party, taking part in Nagy's revolutionary government. A week later, he broke with Nagy over his decision to withdraw from the Warsaw Pact. After Soviet intervention in Hungary, Kádár was selected to lead the country. He ordered Nagy to be executed shortly after coming to power. He gradually moderated, releasing the majority of remaining prisoners of this period in an amnesty in 1963.[citation needed] His leadership was characterized by unrelenting Realpolitik; for a long time, he successfully maneuvered between Moscow's wishes, local interests and the expectations of the Western world. In an interview with a Western journalist, he called himself a "toiler for compromise"