During his studies in Holland, England, and France from 1697 to 1704, Bromelius acquired a thorough knowledge of medicine, anatomy, chemistry, and botany; in 1703 he became a doctor of medicine in Rheims.
Returning to Sweden, Bromell practiced as a physician in Stockholm, where he periodically gave lectures as a professor of anatomy and in 1724 was elected head of the Collegium Medicum; about 1715 he lectured, as a medical professor, in natural history at the University of Uppsala. An accomplished chemist and mineralogist, he became associated with the Board of Mines, where, in 1720, he was named assessor and, in 1724, head of the chemical laboratory.
Bromell earned his scientific renown as a geologist and mineralogist. He was a passionate collector whose great natural history cabinet - some of it inherited from his father - contained a beautiful collection of ore, mineral, and fossil specimens that he described and partly illustrated. In his small Mineralogía (1730), which was also translated into German and which had considerable influence, Bromell classified, to a degree, minerals according to their chemical characteristics and thus became a forerunner of A. F. Cronstedt and later eighteenth-century mineralogists. His "Litographiae Svccanae specimen secundum" (1727-1730) is a pioneer paleontological work that describes a multitude of Swedish animal and plant fossils, including trilobites, ammonites, and corals from Gotland limestone.
Bromelius became a doctor of medicine in 1703 in Reims, and was appointed a member of the Collegium medicum in 1705.
In 1707, Magnus Bromelius married Anna Beata Enhielm, by whom he had several children.