Fiennes was born on 14 April 1911, the son of Gerald Yorke Twisleton-Wykeham-Fiennes and his wife Gwendolen, née Gisborne; part of the Twisleton-Wykeham-Fiennes family, on his father's side he was the great-grandson of Frederick Fiennes, 16th Baron Saye and Sele.
[1] Educated as "Founders' kin" at Winchester College (he was descended from William of Wykeham), Fiennes went up to Balliol College, Oxford, where he read classics; graduating in 1934 with a first-class degree, he won several named scholarships and the Gaisford Prize.
[1][2] Called to the bar in 1936, Fiennes carried out pupillages under F. E. Farrer and J. Neville Gray, and then practised privately before joining the Office of the Parliamentary Counsel in 1939.
Appointed a Companion of the Order of the Bath in 1953, Fiennes was promoted to Knight Commander in 1970; he was a Bencher of the Middle Temple and took silk in 1972.
The Times called Fiennes "unquestionably the ablest draftsman of this century ... his amazing memory was stored with an encyclopaedic knowledge of the law and its history ... he was in the same class as his great Victorian predecessor Lord Thring".