Born in Neuilly-sur-Seine near Paris, Sohn-Rethel came from a family of painters, his father was painter, Alfred Sohn-Rethel (1875–1958) and his mother was Anna Julie, née Michels. His mother was a descendant from the noble Oppenheim family and had influential relations with big business. His paternal grandfather was painter, Karl Rudolf Sohn and his paternal grandmother was painter and singer, Else Sohn-Rethel. As his family did not want him also to become a painter, he was brought up by his uncle, the steel industrialist Ernst Poensgen. On Christmas 1915, he expressed a wish for a copy of Karl Marx "Capital" as a present. He received one and studied it intensively. Thrown out of home, he participated in the anti-war student protest in his first year at Heidelberg University in 1917.