Platt was born in New York City on March 10, 1936, the son of Helen (née Choate) and architect Geoffrey Platt. His maternal great-grandfather was diplomat and lawyer Joseph Hodges Choate. Platt served as U.S. Ambassador Extraordinary and Plenipotentiary to Pakistan, Philippines, Zambia, and as a high level diplomat in Canada, China, Hong Kong, and Japan. He is the former president of the Asia Society in New York City. Platt entered the Foreign Service of the United States in 1959. Prior to that, he was a research assistant at the Washington Center for Foreign Policy Research. From 1959 to 1961, he served as vice consul in Windsor, Ontario, Canada, where his son, the actor Oliver Platt was born. His older son is Adam Platt, New York Magazine restaurant critic, and his younger son is Nicholas Platt, Jr. His wife is Sheila Maynard, a clinical social worker who worked in Islamabad, granddaughter of Arthur Scott Burden and the Hon. Cynthia Burke Roche.
Platt graduated from the prep school St. Paul’s School, Harvard College (B.A., 1957) and Johns Hopkins University School of Advanced International Studies (M.A., 1959). At Harvard, he was a member of the Porcellian Club. He is articulate in Chinese, German, French, and Japanese.
He studied the Chinese language, 1962-1963, at the Foreign Service Institute and in Taichung, Taiwan. In 1964 he was assigned as political officer at the American consulate general in Hong Kong until 1968, when he became China desk officer in the Bureau of East Asian and Pacific Affairs.From 1969 to 1971, Platt was chief of the Asian Communist Areas Division of the Bureau of Intelligence and Research. In 1971-1973, he served as Deputy Director, and then Director, of the Secretariat Staff in the Department of State.As a young diplomat, Platt accompanied President Richard Nixon on the historic trip to Beijing in 1972 that signaled the resumption of relations between the U.S. and China. Platt was assigned as chief of the political section, U.S. Liaison Office, Peking, China, 1973-1974, and then as deputy chief of the political section at the Embassy in Tokyo, Japan, 1974-1977. He returned to Washington to serve as Director for Japanese Affairs in 1977 and then served as a staff member on the National Security Council at the White House, 1978-1980. From 1980 to 1981, he was Deputy Assistant Secretary of Defense, International Security Affairs, Department of Defense, and then returned to the Department of State as Deputy Assistant Secretary, International Organization Affairs, 1981-1982.Platt was appointed Ambassador to the Republic of Zambia in 1982, where he served until 1985, when he was named Special Assistant to the Secretary of State and Executive Secretary of the Department of State.In 1987, he was appointed Ambassador to the Philippines. In 1991, he was appointed Ambassador to Pakistan where he remained until November 1992.