経済協力局
After the war, Longstreet joined the United States Department of State's economic development division. In 1948, he joined the Economic Cooperation Administration and worked in their office in the Netherlands, and later Paris, until 1952. Upon returning to the U.S., in the mid-1950s, he became vice president and manager of the Louisville branch of the Federal Reserve Bank of St. Louis. Next, he left the public sector to become director of management research at Schering Corporation.