ohn Dee's mentoring role in the Frobisher expeditions may have been important, but was largely informal and behind-the-scenes.

John Dee (1527-1609) was educated at the universities of Cambridge and Louvain, where he made contacts that would serve him well in later years. At Cambridge he met men who were to become prominent in the court of Elizabeth I, such as the future Lord Burghley. At Louvain he studied with experts in cosmography and mathematics such as Gemma Frisius, Abraham Ortelius, Gerard Mercator and Oronce Finé, and was also able to acquire texts and instruments not available in England, including mathematical, geographical and historical books and manuscripts, compasses, two Mercator globes and a ring dial by Gemma Frisius. Dee returned to England with knowledge and instruments necessary for planning and undertaking long-distance exploratory voyages.

He put this knowledge at the service of the 1553 expedition seeking a northeast passage and, as a result, became a scientific adviser to the Muscovy Company. Years later, having read Gilbert's unpublished treatise advocating a search for a northwest passage, and learning that one of the Company's agents, Michael Lok, was involved in such a project, Dee showed interest and was again brought in as an adviser in 1576. He gave a crash-course to Frobisher, Hall and others in the mathematical science of navigation, and may have recommended works on cosmography and navigation to be acquired for the voyages. In gratitude, Lok bought Dee some shares in the venture; Dee may have invested a little of his own money too. Dee was one of the commissioners appointed to oversee the conduct of the expeditions and the smelting operations, and probably contributed to the drafting of the instructions governing the latter two voyages.

He acted in a similar advisory role to the Pet-Jackman voyage to the northeast in 1580. His influence also extended to the later (1585-87) northwest voyages of John Davis, who as a child had probably received some instruction from Dee in science and navigation. Dee went so far as to claim credit for initiating the Davis expeditions.

Dee produced several works which established him as a leading geographical adviser and advocate of the concept of a British Empire. He recognized that maritime dominance, colonizing of new lands, and exploitation of mineral resources there, were the key to England gaining the power to withstand or challenge Spain. Brytanici Imperij Limites (The Limits of the British Empire), written late in the 1570s, outlined Dee's belief in Queen Elizabeth's power over most of the seas and a large amount of land in the northern hemisphere. He used written histories, travellers' accounts, ambassadors' letters and family trees to support his conclusion. This text, along with a map which summarized his claims, was presented to Queen Elizabeth and her ministers in 1580.