Upon returning to the U.S., Gellhorn was hired as an investigator for the Federal Emergency Relief Administration, which sent her to report about the impact of the Depression on the United States.
Her reports for that agency caught the attention of Eleanor Roosevelt, and the two women became lifelong friends.
Her findings were the basis of a novella, The Trouble I've Seen (1936).
Gellhorn first met Hemingway during a 1936 Christmas family trip to Key West.
They agreed to travel in Spain together to cover the Spanish Civil War, where Gellhorn was hired to report for Collier's Weekly.
Later, from Germany, she reported on the rise of Hitler and in 1938 was in Czechoslovakia.
She reported on World War II from Finland, Hong Kong, Burma, Singapore, and Britain.
Lacking official press credentials to witness the D-Day landings, she impersonated a stretcher bearer and later recalled, "I followed the war wherever I could reach it."
She was among the first journalists to report from Dachau concentration camp after it was liberated.
After the war, Gellhorn worked for the Atlantic Monthly, covering the Vietnam War, the Six-Day War in the Middle East, and the civil wars in Central America.
Aged 81, she traveled in 1989 to Panama, where she wrote on the U.S. invasion.
Gellhorn published a large number of books, including a collection of articles on war, The Face of War (1959); a novel about McCarthyism, The Lowest Trees Have Tops (1967); an account of her travels (including one trip with Ernest Hemingway), Travels With Myself and Another (1978); and a collection of her peacetime journalism, The View From the Ground (1988).