Rupert was born on the 12th May 1894 the third son of Rear Admiral Sir Thomas Hounsom Butler Fellowes KCB and Margaret Fellowes (nee Jowitt). The family lived at Woodfield Park, Stevenage.
Educated at the United Services College 1903-05, the Royal Naval School, Eltham and Balliol College, Oxford. He became a student of Balliol College, Oxford and on the 19th August 1914 applied for service in an Infantry regiment. The 20 year-old, who stood at 6 feet tall, was, at that time, an undergraduate and had a passion for horse riding.