John Dee was a real-world astronomer, astrologer, occultist and alchemist incorporated into the Cthulhu Mythos by Frank Belknap Long for his short story "The Space-Eaters" (1928) and by H. P. Lovecraft for his short story "The Dunwich Horror" (1929).
In Real Life[]
In real life, Dee was inducted into the service of the crown when Queen Elizabeth took the throne in 1558, acting as her astrological and scientific advisor.
In the Mythos[]
At around the same time he was granted his official positions, he was also given the title of Royal Occultist, in reward for an unrecorded act in service to the crown. This title has remained into at least the first half of the twentieth century (where it was occupied by one Charles St. Cyprian).
At some unknown point in his career, Dee translated his friend Baron Hauptman's Greek copy of the dread Necronomicon into English. This version is known to be imperfect, but still contains a plethora of functional spells. This version is one of the most commonly referenced in the Cthulhu Mythos.