Born into a wealthy merchant family in Gothenburg as the son of Nils Pehrsson Sahlgren and Sara Herwegh, Sahlgren was sent at the age of 16 as an apprentice to the Tietzen & Schröder trading house in Amsterdam, where he learned languages and other aspects of trade. Several additional years of travels on the European continent, to England, and in Sweden, gained him experience, knowledge of natural resources and important contacts. He settled in Gothenburg again on the death of his mother and became a burgess of Gothenburg in 1733.