Magnus was born in Berlin to a Jewish family, his father a wealthy merchant. In his youth he received private instruction in mathematics and natural science. At the University of Berlin he studied chemistry and physics, 1822–27, and obtained a doctorate for a dissertation on tellurium in 1827. His doctoral adviser was Eilhard Mitscherlich. He then went to Stockholm for a year as a visiting research fellow at the laboratory of Jöns Jakob Berzelius (who was a personal friend of Mitscherlich). That was followed by a year in Paris at the laboratory of Joseph Louis Gay-Lussac and Louis Jacques Thénard. Therefore, he had a first-rate education in experimental science when in 1831 he was appointed lecturer in physics and technology at the University of Berlin. In 1834 he became assistant professor, and in 1845 was appointed full professor, and later he was elected the dean of the faculty.