Gellhorn attended Mary Institute and then John Burroughs School, which her mother helped establish so that her daughter could attend a coeducational private high school. She helped found the "John Burroughs Review" in which forty-two of her poems were published. She attended Bryn Mawr University and wrote poetry that was published in the university´s literary magazine, the Lantern. Gellhorn worked at the New Republic, the Albany Times, and sent stories to the St. Louis Post Dispatch.

Gellhorn went to France at the age of 20 and there she wrote her first novel, What Mad Pursuit. She returned to travel around America in 1931. During a job as a field investigator for the Federal Emergency Relief Administration, she wrote about her experiences in The Trouble I´ve Seen, a book of novellas published in 1936. Gellhorn was the first woman correspondent during the Spanish War and also worked on the front lines of World War II. She wrote fiction as well as articles on the war. Her The Face of War, a collection of her wartime articles, was written in 1959. Gellhorn was married three times, including once to Ernest Hemingway.