In 1930 Barnes enlisted in the United States Foreign Service, and was appointed Vice Consul at Bucharest, Romania, and then in Sofia, Bulgaria.
In 1938 he took a position at Russell Sage College in Troy, New York, where he quickly rose to become an associate professor and chair of the English department.
During World War II Barnes served as a civilian consultant to the Joint Chiefs of Staff and as a military information officer with the O.S.S.
After the war (September 1946) Barnes took a position at Dickinson College, as the Thomas Beaver Professor of English Literature and chair of the department.
Barnes left Dickinson officially in 1953 to head the Institution of American Studies at the Free University of Berlin, where he had been since 1951.