Benson was the youngest son of Edward White Benson (Archbishop of Canterbury) and his wife, Mary, and the younger brother of Edward Frederic Benson and A. C. Benson.
Benson was educated at Eton College and then studied classics and theology at Trinity College, Cambridge, from 1890 to 1893.
In 1895, Benson was ordained a priest in the Church of England by his father, who was the then Archbishop of Canterbury.
After his father died suddenly in 1896, Benson was sent on a trip to the Middle East to recover his own health. While there he began to question the status of the Church of England and to consider the claims of the Roman Catholic Church. His own piety began to tend toward the High Church tradition, and he started exploring religious life in various Anglican communities, eventually obtaining permission to join the Community of the Resurrection.
Benson made his profession as a member of the community in 1901, at which time he had no thoughts of leaving the Church of England. As he continued his studies and began writing, however, he became more and more uneasy with his own doctrinal position and, on 11 September 1903, he was received into the Catholic Church. He was awarded the Dignitary of Honour of the Order of the Holy Sepulchre.
Benson was ordained as a Roman Catholic priest in 1904 and sent to Cambridge. He continued his writing career along with his ministry as a priest.
Like both his brothers, Edward Frederic Benson ("Fred") and Arthur Christopher Benson, Robert wrote many ghost and horror stories, collected in The Light Invisible (1903) and A Mirror of Shallott (1907).His novel, Lord of the World (1907), is generally regarded as one of the first modern dystopias (see List of dystopian literature).
As a young man, Benson recalled, he had rejected the idea of marriage as "quite inconceivable". Benson was appointed a supernumerary private chamberlain to the Pope in 1911 and, consequently, styled as Monsignor.
Benson died in 1914 in Salford, where he had been preaching a mission. He was 42. At his request, he was buried in the orchard of Hare Street House, his house in the Hertfordshire village of Hare Street. A chapel, dedicated to St Hugh, was built over the site. Benson bequeathed the house to the Catholic Church as a county retreat for the Archbishop of Westminster. The Roman Catholic church in the nearby town of Buntingford, which he helped finance, is dedicated to St Richard of Chichester, but also known as the Benson Memorial Church.