Farwell graduated from Ohio State University and the University of Chicago (M.A., 1968). He served in World War II as a captain of engineers attached to the Mediterranean Allied Air Force in the British Eighth Army area and later also saw combat in the Korean War. He separated from the military after seven years of active duty.
As a civilian, he became director of public relations and director of administration for Chrysler International from 1959 to 1971. He also served three terms as mayor of Hillsboro, Virginia (1977–81).
He published articles in the New York Times, Washington Post, American Heritage, Harper's, Horizon, Smithsonian Magazine as well as serving as a contributing editor to Military History, World War II, and Collier's Encyclopedia. Farwell also published biographies of Stonewall Jackson, Henry M. Stanley, and Sir Richard Francis Burton.
Farwell gave his papers to the University of Iowa.
- Let's Take a Trip in Our Car [children's book] (1954)
- Walter P. Chrysler (1957)
- The Man Who Presumed: A Biography of Henry M. Stanley (1957)
- Burton: A Biography of Sir Richard Francis Burton (1963)
- Prisoners of the Mahdi (1967)
- Queen Victoria's Little Wars (1972) ISBN 0-060-11222-0
- Mr. Kipling's Army (1981) ISBN 0-393-01386-3 OCLC 6330952
- The Ghurkas (1984) ISBN 0-393-01773-7 OCLC 9575881
- Eminent Victorian Soldiers: Seekers of Glory (1985) ISBN 0-393-01884-9
- The Great War in Africa, 1914–1918 (1986) ISBN 0-393-02369-9 OCLC 13498922
- Armies of the Raj: from the Mutiny to Independence, 1858-1947 (1989) ISBN 0-393-02679-5
- The Great Anglo-Boer War (1990) ISBN 0-060-11204-2 OCLC 1945130
- Ball's Bluff: A Small Battle and Its Long Shadow (1990), McLean, Virginia: EPM Publications; ISBN 0-939009-36-6.
- Stonewall: A Biography of General Thomas J. Jackson (1992) ISBN 0-393-31086-8 OCLC 29534965
- Over There: The United States in the Great War, 1917-1918 (1999)