At the 1906 general election, Fiennes was elected as Member of Parliament (MP) for Banbury and with a brief interruption in 1910, held the seat until the 1918 general election. He was also Parliamentary Private Secretary to Winston Churchill, the then First Lord of the Admiralty from 1912-14 and fought in Flanders and the Dardanelles during World War I.
Created a baronet in 1916, Fiennes left the Commons two years later to become Governor of the Seychelles and was then Governor of the Leeward Islands from 1921-29. In 1894, he had married Florence Agnes Fletcher née Rathfelder (from Constantia, Cape Town) and they had two children: John Eustace (1895–1917, Battle of Arras) and Sir Ranulph Twisleton-Wykeham-Fiennes, 2nd Baronet (1902-1943). Fiennes died in 1943 and his title was inherited by his son who died the same year, the explorer Sir Ranulph Fiennes inherited the title on his birth in 1944.