She began her writing career for The New York Times, where she worked as a staff reporter from 1968 to 1973. After leaving the Times, she continued to work as a freelance journalist for that publication and others, notably covering the Patty Hearst/Symbionese Liberation Army case from 1974-1976 and the Peoples Temple case in 1978. She was also one of the few people to interview reclusive author J. D. Salinger, in 1974.
In 1977, Fosburgh published her first book, Closing Time: The True Story of the Goodbar Murder. A true story that grew out of a 1973 murder case Fosburgh had covered for the New York Times, the book became a bestseller. It was also met with critical acclaim, being selected by the Literary Guild and Doubleday Book Club, and receiving a 1978 Edgar Award nomination for Best Fact Crime book. Truman Capote remarked that the book proved Fosburgh "a skillful, selective reporter and also a literary artist."
Her second book, Old Money (1983), a novel which was understood to be largely autobiographical, about growing up in a wealthy, troubled family, was also a bestseller. Her third book was India Gate (1991), a fictional family saga and mystery involving the children of American expatriates in India. Fosburgh also taught in the University of California, Berkeley, School of Journalism.