He was some time professor of philosophy and theology at Herborn, in Nassau, and afterwards at Weissenburg (present Alba Iulia) in Transylvania, where he remained till his death in 1638. He was a prolific writer, and his Encyclopaedia (1630), the most considerable of the earlier works of that class, was long held in high estimation.
In his The New England Mind, Perry Miller writes about the Encyclopaedia, "It was indeed nothing short of a summary, in sequential and numbered paragraphs, of everything that the mind of European man had yet conceived or discovered. The works of over five hundred authors, from Aristotle to James I, were digested and methodized, including those of Aquinas, Scotus, and medieval theology, as also those of medieval science, such as De Natura Rerum."
Alsted has been called 'one of the most important encyclopedists of all time'.