Kata Pejnović was born on 21 March 1899 in the village of Smiljan in the Kingdom of Croatia-Slavonia, then part of the Austro-Hungarian Empire to a poor family.

She completed her only formal education, elementary school, in 1911, before starting work to help feed her family.

Following the formation of the anti-communist Independent State of Croatia after the Axis invasion of Yugoslavia in April 1941, the Croatian fascists killed her husband and three sons in July.

[1] In the communist party, Pejnović focused on reducing ethnic tensions between Serbs and Croats and women's issues.

Later that year, she was the only woman delegate to the Anti-Fascist Council for the National Liberation of Yugoslavia (Croatian: Antifašističko vijeće narodnog oslobođenja Jugoslavije) in November and Pejnović was elected President of the Antifascist Women's Front (Antifašistički front žena) shortly afterwards.