Julia Kowalska was born in 1918 in Warsaw to a Polish and Jewish family. Her father, David Kowalski, was a famous doctor, and her mother, Anna Kowalska, taught literature at a local school. From an early age, Julia showed a love for books and art, which led her to choose the profession of an artist.

In 1939, when Julia was 21, the German occupation of Poland began. Soon after, her parents were killed in the bombing of Warsaw. Left an orphan, Julia was forced into hiding to avoid becoming a victim of the Intelligenzaktion.