Born at Kalmar in Sweden, Mosander started his career as a physician and became Jöns Berzelius's assistant after a time in the army. He became curator of minerals at the Royal Academy of Science in Stockholm before succeeding Berzelius as secretary. In 1832 he became professor of chemistry and mineralogy at the Karolinska Institute, Stockholm.

Mosander worked chiefly on the lanthanoid elements. These had been known since the discovery of yttrium by Johan Gadolin in 1794 and cerium by Martin Klaproth in 1803. He began by examining the earth from which cerium had been isolated, ceria. From this he derived in 1839 the oxide of a new element, which he called lanthanum, from the Greek meaning ‘to be hidden’. In 1843 he announced the discovery of three new rare-earth elements – erbium, terbium, and didymium. As it happened, didymium was not elementary, being shown in 1885 by Karl Auer von Welsbach to consist of two elements – praseodymium and neodymium.