Francis Walsingham
Elizabeth I's principal secretary and spymaster, who built a network of agents and mail interceptors across Europe.
Walsingham served as principal secretary to Elizabeth I from 1573 to 1590 and ran England's intelligence effort in practice. He kept agents on the Continent and had mail to and from Catholic sympathisers intercepted and examined. His greatest success was the Babington Plot of 1586, when Mary Stuart's letters were read as they passed. Its cryptanalyst, Thomas Phelippes, did most of the reading.