Jack Warner was born in London, Ontario, in 1892. His parents were Jewish immigrants from Poland who spoke mainly Yiddish. Jack was the fifth surviving son of Benjamin Warner a cobbler from Krasnosielc, Poland (then located in the "Russian" part of Poland known as Congress Kingdom), 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). The Warner family had occupied a "hostile world", where the "night-riding of cossacks, the burning of houses, and the raping of women were part of life's burden for the Jews of the 'shtetl'". In 1888, in search of a better future for his family and himself, Benjamin made his way to Hamburg, Germany, and then took a ship to America. The Warner surname was perhaps originally "Wonsal" or "Wonskolaser" Upon arriving in New York City, Benjamin introduced himself as "Benjamin Warner", and the surname "Warner" remained with him for the rest of his life. Pearl Warner and the couple's two children joined him in Baltimore, Maryland, less than a year later. In Baltimore, the couple had five more children, including Abraham (later known as Albert) and Sam Warner.
Benjamin Warner's decision to move to Canada in the early 1890s was inspired by a friend's advice that he could make an excellent living bartering tin wares with trappers in exchange for furs. Their sons Jack and David were born in Ontario. After two arduous years in Canada, Benjamin and Pearl Warner returned to Baltimore, bringing along their growing family. Two more children, Sadie and Milton, were added to the household there. In 1896, the family relocated to Youngstown, Ohio, following the lead of Harry Warner, who established a shoe repair shop in the heart of the emerging industrial town. Benjamin worked with his son Harry in the shoe repair shop until he secured a loan to open a meat counter and grocery store in the city's downtown area.