Larsen worked as an Assistant Professor of History at the University of Washington from 1921 to 1926. From 1926 through 1929, he served in the same capacity at Ohio State University. He was promoted to Associate Professor and continued in that capacity at Ohio State University until 1930. In 1930–1943 he served as an Associate Professor of History at the University of Chicago, then as a Professor (1943–1953, and as a Professor Emeritus from 1953 to 1971. Professor Larsen was the managing editor of "Classical Philology" from 1939 to 1951. From 1951 to 1952 he was chairman of the American Philological Association. He served as the Sather Professor of Classics at the University of California-Berkeley in 1954 and as Visiting Professor of History at Rutgers University in 1956 and 1957. He was also Visiting Professor of History at the University of Texas in 1960 and at the University of Missouri from 1960 to 1971.