Cassel was born in Gross-Glogau, Silesia, Prussia. His father was a sculptor, and his brother David was a well-known rabbi in Berlin and docent at the Hochschule für die Wissenschaft des Judenthums (College for the Study of Judaism). Cassel studied at the Gymnasium of Glogau and Schweidnitz and at the University of Berlin, where he followed with special interest the lectures of the historian Leopold von Ranke. In 1849 he edited in Erfurt "Constitutionelle Zeitung", and in 1850-56 "Erfurter Zeitung", in a royalist spirit. He later received a doctor of divinity degree from Vienna.