After earning his Ph.D. from Cornell University in 1949, Lyon accepted a position in the history department at the University of Colorado. In 1954 Lyon compiled and edited a volume of Carl Stephenson's finest articles,
Medieval Institutions: Selected Essays by Carl Stephenson, to recognize his mentor's scholarly career. From 1956 to 1959, Lyon was an associate professor of history at the University of Illinois. Together with other professors from the department of history at Illinois, Lyon co-wrote ''A History of the World'
in 1960. Additionally, Lyon wrote "The Middle Ages in Recent Historical Thought"
published in 1959. During this time, Lyon also began his book entitled A Constitutional and Legal History of Medieval England'' published in 1960. Lyon recognized trade required a mature legal system. From 1959 through 1965, Lyon was a professor of history at the University of California at Berkeley. Lyon also served as assistant dean to the College of Arts and Sciences from 1959 through 1965. While at Berkeley, Lyon revised
Medieval History, Europe from the Second to the Sixteenth Century by Carl Stephenson. From 1965 until his retirement in 1986, Lyon was the Barnaby C. and Mary Critchfield Keeney Professor of History at Brown University in Providence, RI. Lyon also chaired the department of history at Brown University from 1968 through 1975. During this time, Lyon's notable books included:
Medieval Finance. A Comparison of Financial Institutions in Northwestern Europe,
A History of the Western World,
Studies of West European Medieval Institutions, and
The Emergence of Common Law and Parliament. While at Brown, Lyon also researched the life of Henri Pirenne and the
Annales School of History. This research resulted in published editions of Pirenne's correspondence with
Marc Bloch and
Karl Lamprecht, a major article on
Maurice Prou’s ties to Pirenne, and the biography entitled
Henri Pirenne: A Biographical and Intellectual Study. Working as a team, Lyon and his wife, Mary, translated
The Wardrobe Book of William de Norwell 12 July 1338 to 27 May 1340. Deciphering Latin abbreviations into full Latin, Lyon and his wife gave future scholars access to the account books of
Edward III revealing his military, political, financial and logistical strategy during the
Hundred Years’ War. == Post-Retirement ==