In 1941 he migrated to the United States and became a citizen. He did his doctoral work in philosophy at Northwestern University in 1946. He taught at the University of Berlin and at Lake Forest Academy near Chicago. He was a professor of philosophy at the College of Wooster (1945 - 1948) and at Ohio State University (1948 - 1956) and was a visiting professor at the Massachusetts Institute of Technology (1955 - 1956) and at Yale University in 1966. Hartman was a Research Fellow and Exchange Professor at the National University of Mexico from 1956 - 1957, and from 1957 until his death he was a research professor of philosophy there. From 1968 until his death Hartman also held an appointment as a professor of philosophy at the University of Tennessee. In the late 1950s he was among the first members of the Society for General Systems Research.