Born into a Jewish family in November 1913 in Vilna (Wilno, Vilnius), then in Russian Empire, he and his family left for Harbin, Manchuria (northern China), in 1916. There Leo joined a Zionist group and studied Jewish history at a Jewish primary school, and then studied at a Russian secondary school. When Japan occupied Manchuria in 1931, he went to Shanghai where attended a British school, and also served in the Jewish Shanghai Volunteer Corps.