He received his doctorate in law from the Sorbonne (1949), and returned to Harvard again to obtain a graduate degree in international relations (M.A., 1950).

He interrupted his education and served in the United States Army from 1944 to 1946, utilizing his language skills with the Office of Military Intelligence.

Dean was trained at Fort Belvoir, after which he was sent to a top-secret intelligence site named Post Office Box 1142.

After he left he was offered a job in the newly formed Central Intelligence Agency, which he refused on advice from his father.

From 1953 to 1956 he was assistant economic commissioner with the International Cooperation Administration in French Indo-China with accreditation in Saigon, Phnom Penh, and Vientiane.

[6] Also in 1963 Dean was an adviser to the U.S. delegation to the 18th Session of the United Nations General Assembly, and during 1964–1965 he was an international relations officer in the NATO section of the Department of State.

While in Da Nang, South Vietnam, he helped to protect the Cham Museum for which he was officially thanked in 2005 by the Vietnamese and French authorities.

In August 1980, while serving as ambassador to Lebanon, Dean was the target of an assassination attempt, which, evidence dictated, was directed by Israel.

Undoubtedly using a proxy, our ally Israel had tried to kill me.Dean's suspicions that Israeli agents may have also been involved in the mysterious aircraft crash in 1988 that killed Muhammad Zia-ul-Haq, the president of Pakistan, led to a decision in Washington to declare him mentally unfit, which forced his resignation from the foreign service after a thirty-year career.

In India, Dean helped bring about the withdrawal of Soviet troops from Afghanistan according to an agreed time table.