Grandson of United States Senator James B. Eustis, Bohlen joined the State Department, learned Russian and became a Soviet specialist, working first in Riga, Latvia. In 1934 he joined the staff of the embassy in Moscow. In 1940–41 he worked in the American Embassy in Tokyo, and was interned for six months before release by the Japanese in mid-1942. He worked on Soviet issues in the State Department during the war, accompanying Harry Hopkins on missions to Stalin. He worked closely with President Franklin D. Roosevelt and was Roosevelt's interpreter at the Tehran Conference (1943) and the Yalta Conference (1945).
In 1946 he disagreed with his friend George Kennan on how to deal with the Soviets (Harper 1995). Kennan proposed a strategy of containment of Soviet expansion, while Bohlen was more cautious and recommended accommodation, allowing Stalin to have a spheres of influence in Eastern Europe. Bohlen paid more attention to liberal public opinion, since he believed domestic influence in a democracy was inevitable (Ruddy 1986). When George C. Marshall became Secretary of State in 1947, Bohlen and Dean Acheson were key advisers.
In 1953 President Dwight D. Eisenhower named Bohlen ambassador to the Soviet Union; he was confirmed by a vote of 74–13 despite the criticisms made by Senator Joe McCarthy. Bohlen did not enjoy a good relationship with Soviet leaders, or with Dulles. He was demoted in 1957 to become ambassador to the Philippines (1957–59). He was ambassador to France (1963–68).
In his 1973 memoirs, Witness to History, he reveals that on the morning of August 24, 1939, he visited the Third Reich diplomat Hans von Herwarth and received the full content of the secret protocol to the Molotov-Ribbentrop Pact, signed the day before (Bohlen 1973). The secret protocol contained an understanding between Adolf Hitler and Joseph Stalin to split Central Europe, the Baltic region, and Finland between their nations. U.S. President Franklin D. Roosevelt was urgently informed. The United States did not convey this information to any of the concerned governments in Europe. A week later the plan was realized with the German invasion of Poland, and World War II was commenced.
In 2006, Bohlen was featured on a United States postage stamp, one of a block of six featuring prominent diplomats.