Born in Ulm, Germany, Frenshemius was a scholar of ancient Roman history and was named to a chair at the University of Uppsala in 1641, and then five years later he was called to Queen Christina's court in Stockholm as her librarian, historiographer, and literary major-domo.

The story has it that the French ambassador Chanut suggested that the queen might best facilitate her reading of Descartes by having Freinshemius read it with her, with the result that since the latter felt the same need of a reading companion, Chanut also wound up reading Descartes. The queen had had a particular interest in the sovereign good and chose it as a topic for a public speech at the university, but she found Freinshemius's account superficial and wanted to know Descartes’ view of the matter. This might have led to Chanut's correspondence with Descartes on the topic of love in 1647 and eventually to his invitation to Stockholm. Concerned about how as a foreign, Catholic notable he would be received at court, Descartes wrote to Freinshemius about what he should do (AT V 362–63). (The reassurances he received, though honest and well intended, proved not to be entirely accurate, though through no apparent fault of Freinshemius.) According to Descartes, Freinshemius was present for the early-morning lessons in philosophy that he gave Christina in her library (AT V 466, CSMK 383).

After five years at the court, he left Sweden, because of the climate, and went to Heidelberg, where he died, with the titles of counselor to the elector of the Rhine Palatinate, brother of Princess Elisabeth, and honorary professor at the University.

See also Chanut, Hector-Pierre; Christina, Queen of Sweden; Elisabeth, Princess of Bohemia