Reynolds briefly studied in Paris, and then returned to Milwaukee to work as a real estate salesman.

By 1931, Reynolds had returned to the United States, where he was appointed an assistant professor of medieval and economic history at the University of Wisconsin.

When visiting Genoa in 1938, Reynolds established a life-long friendship with the Italian historian Robert Sabatino Lopez, with whom he would later write a seven volume history of Italy.

He earned a certificate of merit from Dwight D. Eisenhower on behalf of the Supreme Headquarters Allied Expeditionary Force.

In 1948 he helped form the University of Wisconsin's Integrated Liberal Studies program.

Reynolds was the author of numerous books and scholarly articles on the economic history of the Middle Ages.