 Beloff was born on 2 July 1913 at 21 York House, Fieldway Crescent, Islington, London and was the oldest child of a Jewish family who had moved to England in 1903 from Russia.He was the elder son in a family of five children of merchant Semion (Simon) Beloff (born Semion Rubinowicz) and his wife Maria (Marie) Katzin.His sister Anne later married German-born Nobel Prize–winning biochemist Ernst Boris Chain in 1948.