Jack Lang was born to Roger Lang and Marie-Luce Bouchet in Mirecourt, in the département of Vosges. His father's family were a secular, assimilated, well-to-do Jewish family based in Nancy. Roger Lang was the commercial manager of the family business which was founded by Jack's grandfather Albert. Roger and Albert were both freemasons. Jack's mother, Marie-Luce Bouchet, a Catholic, was born in 1919 as the daughter of Emile Bouchet, who died in 1926, and Berthe Boulanger, a nurse who was also a freemason.