[1][2] Kubilius was born in Fermos village, Eržvilkas county, Jurbarkas District Municipality, Lithuania on 27 July 1921.

[3][5] He received the Doctor of Sciences degree (habilitation) in 1957 from the Steklov Institute of Mathematics in Moscow.

[7]: 190  Česlovas Masaitis attributes Kubilius's success to "his ability to manipulate within the complex bureaucratic system of the Soviet Union and mainly because of his international recognition due to his scientific achievements.

[8] According to some sources Lithuanian partisans suggested him to continue studies and stay alive to work for Lithuania in the future.

In 1952 Kubilius became an employee of the Lithuanian Academy of Sciences in the Physics, Mathematics and Astronomy Sector.

[3] In addition to his scientific and administrative work, Kubilius was a member of the Seimas (Lithuanian parliament) from 1992 to 1996.