Known best as a sixteenth-century Hermetic philosopher, he also studied alchemy, the occult, astronomy, and mathematics. In addition, he worked as a personal astrological consultant to Queen Elizabeth I of England.
He studied at St. John's College, Cambridge and later joined a London trade association called the Worshipful Company of Mercers.
In 1555, he was charged with treason (and later exonerated of the charge) for an astrological prediction he made about the reign of Queen Mary.
His second marriage, to the much-younger Jane Fromand, produced eight children. He was the only child of Jane Dee and Welsh merchant Rowland Dee.
He and sixteenth-century astronomer Tycho Brahe were contemporaries and friends.