After resigning from the Army, Porter became vice president of the Pullman Palace Car Company, and later, president of the West Shore Railroad. He was U.S. Ambassador to France from 1897 to 1905, paying for the recovery of the body of John Paul Jones and sending it to the United States for re-burial. He received the Grand Cross of the Legion of Honor from the French government in 1904. In addition to "Campaigning with Grant", he also wrote "West Point Life" (1866).