He worked as a chemist for the Clinton Laboratories (now Oak Ridge National Laboratory) during the World War II Manhattan Project, engaged in separating, identifying and characterizing the radioactive elements produced by nuclear fission. In 1945, he, together with Jacob A. Marinsky and Charles D. Coryell, isolated the previously undocumented rare-earth element 61 (promethium). Marinsky and Glendenin produced it both by extraction from fission products and by bombarding neodymium with neutrons. They isolated it using ion-exchange chromatography. Publication of the finding was delayed until later due to the war. In September 1947, Marinsky and Glendenin announced the discovery at a meeting of the American Chemical Society. Upon the suggestion of Coryell's wife, the team named the new element for the titan god Prometheus, who stole fire from the gods and was punished for the act by Zeus. They had also considered naming it "clintonium" for the facility where it was isolated.