The nephew of Winston Churchill, Romilly was born in 1918. Educated at Wellington College he caused a stir when he declared he was a pacifist and with his brother, Giles Romilly, refused to join the Officer Training Corps.
The brothers also distributed communist leaflets in the school and began publishing a left-wing journal, Out of Bounds: Public Schools’ Journal Against Fascism, Militarism and Reaction. In the first issue Romilly stated that the journal would "openly champion the forces of progress against the forces of progress against the forces of reaction on every front, from compulsory military training to propagandist teaching." The journal soon had a circulation of over 3,000 copies.
In 1934 the Daily Mail wrote an article about the activities of the Romilly brothers under the headline: "Red Menace in Public Schools! Moscow Attempts to Corrupt Boys". Soon afterwards the fifteen run away from school and went to work for a Communist bookshop in London. He also established a centre for other boys who had run away or had been expelled from public schools.
Romilly was eventually arrested and after his mother had told the judge that he was uncontrollable he was sentenced to a six-week term in a Remand Home for delinquent boys. On his release Romilly joined forces to publish the book Out of Bounds: The Education of Giles and Esmond Romilly (1935). The book received good reviews and the Observer commented on its "considerable intelligence, modesty, and tolerance, a series of clear, humorous, and lively pictures of schools, boys, masters and parents"
On the outbreak of the Spanish Civil War Romilly joined the International Brigades. As the British Battalion had not yet been formed Romilly and 15 other Englishmen were attached to the German Thaelmann Battalion. Romilly fought in the defence of Madrid but by December 1936 all but two of the English group had been killed or seriously wounded. The following month, suffering from dysentery, he was sent home.
In February 1937 Romilly returned to Spain as a journalist with the News Chronicle. His girlfriend, Jessica Mitford, went with him and they married in June 1937. While on honeymoon Romilly wrote Boadilla, an account of his experiences in the Spanish Civil War.
When Romilly returned to England he found work as a copywriter for a small advertising agency in London, whereas Jessica was employed in market research. Along with his wife Romilly became involved in the struggle against the British Union of Fascists.
In 1939 Mitford and Romilly went to the United States. On the outbreak of the Second World War Romilly went to Canada and joined the Royal Canadian Air Force but was killed in November 1941 during a bombing raid over Nazi Germany.