Jacob Warner (as he was named at birth) was born in London, Ontario, in 1892. His parents were Polish-Jewish immigrants from Congress Poland (then part of Russian Empire), who spoke mainly Yiddish. Jack was the fifth surviving son of Benjamin Warner (originally "Wonsal" or "Wonskolaser"), a cobbler from Krasnosielc, and his wife, the former Pearl Leah Eichelbaum. Following their marriage in 1876, the couple had three children in Poland, one of whom died at a young age. One of the surviving children was Jack's eldest brother, Hirsch (later Harry).