Born in Virginia, Johnson was educated at Harvard University. He was instructor in history at Bowdoin College and Williams College, becoming Associate Professor at the latter in 1938. He took part in 1942 to join the wartime State Department. Johnson became chief of the department's Division of International Security Affairs in 1945, having served as acting chief from 1944.
From 1950 to 1971 he was president of the Carnegie Endowment for International Peace. He was a member of the board of trustees of the World Peace Foundation in the early 1950s. From 1954, he was inaugural American secretary of the annual Bilderberg conference which discusses matters relating to European-American relations.
Johnson was Vice President of the International Institute for Strategic Studies from 1965 to 1981.