János Kádár was born as János Csermanek in Fiume, Austro-Hungary (today Rijeka, Croatia) as the son of the soldier János Krezinger (German aka Kressinger) and Borbála Csermanek (Slovak), who was from the little town Ó-Gyalla, Hungary (today Hurbanovo, Slovakia). Kádár spent his first 6 years with foster parents in Kapoly, Somogy County, until reunited in Budapest with his mother, who worked occasionally as a washerwoman and sent him to school until he was 14. (He met his biological father, who lived as a small landowner, and his three half-brothers only in 1960). He apprenticed as a typewriter mechanic, joined the trade union's youth group at 17, and joined the illegal Hungarian Communist Party in 1931, and was subsequently arrested several times for unlawful political activities: he was sentenced to two years imprisonment in 1933. Later, as cover for his illegal communist activities, János Csermanek joined the Hungarian Social Democratic Party and sat on its Budapest branch committee.

He was arrested in 1937 by the Horthy regime and was sent to prison for three years. On his release, he ran the underground communist movement together with his friend László Rajk, from 1943 under the pseudonym János Kádár (In Hungarian kádár mean scooper). In 1944 while trying to cross the border into Serbia, in order to make secret contact with Tito's partisans, he was arrested and dispatched with a transport of Jews to Mauthausen concentration camp. On the way at Komarno while temporarily transferred to the town's prison, he managed to escape and went back to Budapest. Between 1943 and 1945 he was the first secretary of the Communist Party, and between 1943 and 1944 he led its legal cover organization, the Peace Party.

After the occupation of Hungary by USSR and the comeback of the Moscow branch of the leadership of the Communist Party, Kádár was appointed deputy head of Budapest's new police. In 1946, he was elected Deputy Secretary-General of the Hungarian Communist Party. In 1949, he succeeded László Rajk as Minister of the Interior. Rajk was appointed Minister for Foreign Affairs by the Communist Party leader Mátyás Rákosi when he had already been secretly chosen as the chief defendant of a "show trial," to be staged by Rákosi in Hungary. This trial was similar to the show trials initiated by Stalin in the Soviet Union. Rajk and "his spy ring" were accused of conspiring with Marshal Tito, President ofYugoslavia, and were executed. In a Machiavellian scheme, Rákosi put Kádár, who was friends with both Rajk and his wife Julia, in the Interior Minister's position to make sure Kádár was visibly involved in Rajk's trial. In fact, the State Protection Authority (ÁVH), which was in charge of the investigation, took its orders directly from Rákosi; but as interior minister, Kádár condemned Rajk's "crimes", tried to force a confession out of him and attended his execution. Only a year later, Kádár found himself the defendant in a show trial of his own - on false charges of having been a spy of Horthy's police. This time it was Kádár who was beaten by the security police and urged to "confess." He was found guilty, and sentenced to life imprisonment. His incarceration included three years of solitary confinement, conditions far worse than he suffered while imprisoned under the Horthy regime. He was released in July 1954, after the death of Stalin and the appointment of Imre Nagy as Prime Minister in 1953.

Kádár accepted the offer to act as party secretary in the heavily industrialised 13th district of Budapest. He rose to prominence quickly, building up a large following amongst workers who demanded increased freedom for trade unions. Nagy began a process of liberalisation, removing state controls over the press, releasing many political prisoners, and expressing wishes to withdraw Hungary from the Warsaw Pact. He formed a coalition government. Although the Soviet leaders issued a statement that they strived to establish a new relationship with Hungary on the basis of mutual respect and equality, in the first days of November, the Presidium of the Soviet Communist Party took a decision to crush the revolution by force. In the meantime, the Hungarian Communist Party decided to dissolve itself and to reorganize the party under the name of Hungarian Socialist Workers' Party. On October 25, 1956 Kádár was elected Secretary-General. He was also a member of the Imre Nagy Government as Minister of State. On the 1st of November, Kádár, together with Ferenc Münnich left Hungary for Moscow with the support of the Soviet Embassy in Budapest. There the Soviet leaders tried to convince him that a "counter-revolution" was unfolding in Hungary that must be put to an end at any cost. Despite his opposition to leaving the Warsaw Pact decided by Nagy, allegedly he first resisted the pressure and argued that the Nagy government did not wish to abolish the Socialist system. He yielded to pressure only when the Soviet leaders informed him that the decision had already been taken to crush the revolution with the help of the Soviet troops stationed in Hungary and that the old Communist leadership would be sent back to Hungary, were he not willing to assume the post of Prime Minister in the new government. The Soviet tanks moved into Budapest to crush the revolution at dawn on November 4. The proclamation of the so-called Provisional Revolutionary Government of Workers and Peasants, headed by Kádár, was broadcast from Szolnok the same day.

He announced a "Fifteen Point Programme" for this new government:

1. To secure Hungary's national independence and sovereignty
2. To protect the people's democratic and socialist system from all attacks
3. To end fratricidal fighting and to restore order
4. To establish close fraternal relations with other socialist countries on the basis of complete equality and non-interference
5. To cooperate peacefully with all nations irrespective of form of government
6. To quickly and substantially raise the standard of living for all in Hungary
7. Modification of the Five Year Plan, to allow for this increase in the standard of living
8. Elimination of bureaucracy and the broadening of democracy, in the workers' interest
9. On the basis of the broadened democracy, management by the workers must be implemented in factories and enterprises
10. To develop agricultural production, abolish compulsory deliveries and grant assistance to individual farmers
11. To guarantee democratic elections in the already existing administrative bodies and Revolutionary Councils
12. Support for artisans and retail trade
13. Development of Hungarian culture in the spirit of Hungary's progressive traditions
14. The Hungarian Revolutionary Worker-Peasant Government, acting in the interest of our people, requested the Red Army to help our nation smash the sinister forces of reaction and restore order and calm in Hungary
15. To negotiate with the forces of the Warsaw Pact on the withdrawal of troops from Hungary following the end of the crisis

The 15th point was withdrawn after pressure from the USSR that a 200,000 strong Soviet detachment be garrisoned in Hungary. This development allowed Kádár to divert huge defence funds to welfare.

Nagy, along with Georg Lukács, Géza Losonczy and László Rajk's widow, Julia, fled to the Yugoslav Embassy. Kádár promised them safe return home at their request but failed to keep this promise as the Soviet party leaders decided that Imre Nagy and the other members of the government who had sought asylum at the Yugoslav Embassy should be deported to Romania. Later on, a trial was instituted to establish the responsibility of the Imre Nagy Government in the 1956 events. Although it was adjourned several times, the defendants were eventually convicted of treason and conspiracy to overthrow the "democratic state order". Imre Nagy, Pál Maléter and Miklós Gimes were sentenced to death and executed on June 16, 1958. Geza Losonczy and Attila Szigethy both died in prison under suspicious circumstances during the court proceedings.

Kádár assumed power in a critical situation. The country was under Soviet military administration for several months. The fallen leaders of the Communist Party took refuge in the Soviet Union and were planning to regain power in Hungary. The Chinese, East German, and Czechoslovak leaders demanded severe reprisals against the perpetrators of the "counter-revolution." Despite the distrust surrounding the new leadership and the economic difficulties, Kádár was able to normalize the situation in a remarkably short time. This was due to the realization that, under the circumstances, it was impossible to break away from the Communist bloc. The people realized that the promises of the West to help the Hungarian revolution were unfounded and that the logic of the Cold War determined the outcome. Hungary remained part of the Soviet sphere of influence with the tacit agreement of the West. Though influenced strongly by the Soviet Union, Kádár enacted policy slightly contrary to that of Moscow, for example, allowing considerably large private plots for farmers of collective farms.

In notable contrast to Rákosi, Kádár declared that "he who is not against us is with us." He gradually lifted Rákosi's more draconian measures against free speech and movement, and also eased restrictions on art and literature. Hungarians had much more freedom than their Eastern Bloc counterparts to go about their daily lives. While his regime was not nearly as harsh as other Communist regimes (and certainly less so than the first seven years of out-and-out Communist rule in Hungary), it wasn't a liberal one either. The Communists maintained absolute control over the government and also encouraged citizens to join party organizations. The secret police, while operating with somewhat more restraint than in other Eastern Bloc countries (and certainly in comparison to the Rákosi era) were nonetheless a feared tool of government control. The media remained under censorship that was considered fairly onerous by Western standards, but far less stringent than was the case in other Communist countries.

As a result of the relatively high standard of living, and more relaxed travel restrictions than that of other Eastern Bloc countries, Hungary was generally considered one of the better countries in which to live in Eastern Europe during the Cold War. The dramatic fall in living standards after the fall of Communism led to some nostalgia about the Kádár era. However, the relatively high living standards had their price in the form of a considerable amount of state debt left behind by the Kádár régime. As mentioned above, the regime's cultural and social policies were still somewhat authoritarian; their impact on contemporary Hungarian culture is still a matter of considerable debate.

During Kádár's rule, tourism increased dramatically, with many tourists from Canada, the USA, and Western Europe bringing much needed money into Hungary. Hungary built strong relations with developing countries and many foreign students arrived. The "Holy Crown" (referred to in the media as the "Hungarian Crown", so as to prevent it carrying a political symbolism of the Horthy régime or an allusion to Christianity) and regalia of Hungarian kings was returned to Budapest by the United States in 1978.

Kádár was known for his simple and modest lifestyle and had a strong aversion against corruption or ill-doing. His only real hobby was chess.

Kádár was awarded the Lenin Peace Prize (1975-76). He was also awarded the title Hero of the Soviet Union on April 3, 1964.

János Kádár held power in Hungary until 1988, when he resigned as General Secretary mainly due to mounting economic difficulties and his own ill-health. At a party conference in May 1988, he was replaced as General Secretary by Prime Minister Károly Grósz who strove to continue Kádár's policies in a modified and adjusted form adapted to the new circumstances. Kádár was named instead to the rather ceremonial position of Party President. He did not wish to be re-elected to the Political Committee, the most important decision-making body of the party. In early 1989, as Grósz and his associates in turn were being sidelined by a faction of "radical reformers" who set out to dismantle the socialist system, Kádár, now visibly senile, was removed completely from political office, dying not long afterwards.

Kádár was generally known as one of the more moderate East European Communist leaders. While he remained loyal to the Soviet Union in foreign policy, based on the hard lessons of the 1956 uprising, his intent was to establish a national consensus around his policies at home. He was the first East European leader to develop closer links with the Social Democratic parties of Western Europe. He tried to mediate between the leaders of the Czechoslovak reform movement of 1968 and the Soviet leadership to avert the danger of a military intervention. When, however, the decision was taken by the Soviet leaders to intervene in order to suppress the Prague Spring, Kádár decided to participate in the Warsaw Pact operation.

Kádár's grave at the Kerepesi Cemetery in Budapest was vandalized on May 2, 2007; a number of his bones, including his skull, were stolen, along with his wife Mária Tamáska's urn. A message reading "murderers and traitors may not rest in holy ground 1956-2006" was written nearby. The two dates refer to the Hungarian Revolution of 1956 and the 2006 protests in Hungary. This act was greeted with universal revulsion across the political and societal spectrum in Hungary. Police investigations focused on extremist right-wing hooligans groups which had been aspiring to "carry out an act that would create a big bang."