Jančaitytė was born on 3 January 1911 in Žečkalniai [lt] in the present-day Šakiai District Municipality.
[1] Her brother Alfonsas Jančaitis was also a communist who joined the 16th Rifle Division of the Red Army during World War II.
[3] After the Soviet occupation of Lithuania in June 1940, Jančaitytė became head of the communist district committee in Šakiai.
When Germany invaded the Soviet Union in 1941, she retreated from Lithuania and worked at the local communist committee in Soligalichsky District, Kostroma Oblast.
[2] However, she was defended by First Secretary Antanas Sniečkus and the reprimand did not interrupt her communist career.