FitzGerald was the daughter of New York lawyer Desmond FitzGerald and socialite Marietta Peabody. As a teenager, she wrote voluminous letters to Senator Adlai Stevenson expressing her opinion on many subjects, a reflection of her deep interest in world affairs. FitzGerald is married to James P. Sterba, a former writer for The Wall Street Journal. The couple live in New York City and Maine, about which Sterba wrote in his book Frankie's Place.
FitzGerald is best known for her book, The Vietnamese and the Americans in Vietnam (1972), which was met with great acclaim when it was published and remains one of the most notable books about the Vietnam War. She was awarded both a Pulitzer Prize and the National Book Award for the book.
On April 29, 1975, FitzGerald predicted to John Pitney that the new Vietnamese regime would shortly host free, multi-party elections. Such elections never materialized.
FitzGerald's subsequent volumes include America Revised, a highly critical review of high school history textbooks (1979); Cities on a Hill (1987); Way Out There in the Blue: Reagan, Star Wars and the End of the Cold War (2000); Rewriting American history, a short article in The Norton Reader; and Vietnam: Spirits of the Earth (2002).
FitzGerald's writing has also appeared in The New Yorker, and the New York Review of Books, The New York Times Magazine, Esquire, Architectural Digest, and Rolling Stone. She serves on the editorial boards of The Nation and Foreign Policy, and is vice-president of International PEN.