Because of his political involvement, he was expelled from university in 1937 and started working as a journalist for the newspaper Független Újság in Debrecen and the social-democratic Budapest daily Népszava.

[1] During World War II, Kállai was involved in the resistance against the pro-German regime of Miklós Horthy; he was arrested in July 1942 for participation in illegal demonstrations, but released in November of the same year due to lack of evidence.

In September 1944, he participated in the refounding of the MKP, and served as representative of the party to the Hungarian National Independence Front.

[1] In 1945, Kállai was elected a member of the Central Committee of the MKP and made a Secretary of State in the first post-war government.

Kállai held this office until April 1951, when he was arrested on trumped-up charges and sentenced to life imprisonment in a secretly-held trial.