Born in Kalmar, Mosander attended school there until he moved to Stockholm with his mother in 1809.  In Stockholm, he became an apprentice at the Ugglan pharmacy. He took his pharmacy examination in 1817, but had an interest in medicine and joined the Karolinska Institute in 1820. He passed his medical examination in 1825.  He worked in the laboratory of Jöns Jakob Berzelius and became a close friend of fellow student Friedrich Wöhler.