His career as a schoolteacher was broken up by World War II, during which he served with the United States Army Air Forces from 1942 to 1946.
After the war, Klein remained a member of the Air Force Reserve, retiring with the rank of lieutenant colonel in 1977.
Klein continued teaching at the high school level as he worked toward a doctorate from Columbia University.
Klein was subsequently named dean of the College of Liberal Arts and Sciences at Long Island in 1962.
Klein left for a deanship of graduate studies and research at Fredonia College in 1966, then moved to the University of Tennessee at Knoxville in 1969.