Weisz was born on 7 March 1970 in Westminster, London, and grew up in Hampstead Garden Suburb. Her father, George Weisz (1929–2020), was a Hungarian mechanical engineer. Her mother, Edith Ruth (born Teich; 1933–2016), was a teacher-turned-psychotherapist originally from Vienna. Her parents both emigrated to the United Kingdom as children around 1938, prior to the outbreak of World War II, in order to escape the Nazis. George Weisz was from a Jewish family. Edith Weisz's paternal ancestry was Austrian-Jewish; her maternal ancestry was Italian Roman Catholic. The scholar and social activist James Parkes helped her mother's family to leave Austria for England. Weisz's mother was raised in the Catholic church, and formally converted to Judaism upon marrying Weisz's father. Weisz's maternal grandfather was Alexander Teich, a Jewish activist who had been a secretary of the World Union of Jewish Students. Her younger sister, Minnie Weisz, is a visual artist.