As an author on important bibliographic works on physicians and philosophers, Melchior Adam is perhaps the first source on Wier's life. His notes on the life of Wier, published in 1620, became the foreword to the 1659 edition of Wier's Opera Omnia. Indeed, Wier himself provides significant biographical information throughout his works; he was born in 1515 in Grave, Northern Brabant, as he tells us in the De praestigiis. His birthplace, bordering areas of different political and cultural influences, explains the reason behind the variety of spellings of his name that are in use: Weyer in the German form, Wier in the Dutch form, and Wierus in Latin. He was also known by the Latin surname Piscinarius as the Latin term piscina is translated as wiert, which resembles his own surname.

There is disagreement amongst Wier's biographers on some points, such as the social condition of his family. His epitaph defines him as being of noble origin, an understanding shared by Cobben, while, according to Axenfeld, Wier was from a family plébéienne et très honnête. Evidence of the social standing of Wier's family can also be seen in the excellent level of education attained by him.

Wier's interest in demons was present from infancy, as he shows in an anecdote told in his works: while living at home with his family, he had an encounter with a household god, one of the Lares familiares. The use of pagan figures for Christian purposes was by no means uncommon and had been an established use from Renaissance humanist times. In this case, Wier used the encounter to prove the existence of demons.