 Douglas was born Issur Danielovitch () in Amsterdam, New York, the son of Bryna "Bertha" (née Sanglel; 1884–1958) and Herschel "Harry" Danielovitch (c. 1884–1950; citations regarding his exact year of birth differ).His parents were Jewish immigrants from Chavusy, Mogilev Region, in the Russian Empire (present-day Belarus), and the family spoke Yiddish at home.His father's brother, who immigrated earlier, used the surname Demsky, which Douglas's family adopted in the United States.