 From 1942 to 1951 he worked at the U.S. Department of Agriculture, where he worked on breeding forage crops and improving the grazing quality of rangelands in Oklahoma.In 1951 Harlan became a university lecturer and researcher, working first as a professor of agronomy at Oklahoma State University at Stillwater, Oklahoma, where he refused to sign an oath of loyalty, and later as a professor of plant genetics at the University of Illinois Urbana-Champaign.He co-founded the Crop Evolution Laboratory there in 1966.