For the five years following his graduation in 1949, Thetford worked as a research psychologist in both Chicago, and later in Washington, D.C. He was a research psychologist in the Institute for Psychosomatic & Psychiatric Research & Training at the Reese Hospital in Chicago from 1949-1951 and a senior psychologist for the United States government in Washington, DC from 1951-1954. In 1953 he was a consultant in Beirut, Lebanon at the Foreign Service Institute. He spent 1954 and 1955 as the director of clinical psychology at The Institute of Living in Hartford, Connecticut. From 1955 to 1957 he was an assistant professor of psychology at Cornell University.