In 1770, he entered Uppsala University as a freshman (Carolus Linnaeus died in 1778). After receiving the bachelor's degree, he worked in Gothenburg as curator of the private natural museum and botanic garden of K. Alströmer. In 1786 he received a medical doctor's degree at the University of Kiel, Germany. In 1787 he started to teach medicine and botany at the university in Academy of Åbo (today's University of Helsinki).
Dahl was a renowned botanist, the publisher of Observationes botanicæ circa Systema vegetabilium divi a Linné.