In June 1852, he was appointed a deputy lieutenant for Oxfordshire. Fiennes also served in the army with the Queen's Own Oxfordshire Hussars, enlisting in 1852 as a cornet. He was promoted to lieutenant in April 1858 and captain in April 1860. He succeeded his father as 17th Baron Saye and Sele upon his death in May 1887. Fiennes was later present at the Coronation of Edward VII at Westminster Abbey in 1902. Among the offices was a county alderman and justice of the peace for Oxfordshire, in addition to being a justice of the peace for Warwickshire, Middlesex and Westminster. Fiennes died in October 1907 at Broughton, Oxfordshire. He was succeeded as Baron Saye and Sele by his son, Geoffrey Twisleton-Wykeham-Fiennes.