Roskill's oldest brother was Sir Ashton Roskill QC; another older brother Stephen was a Royal Navy officer and historian. He was educated as an exhibitioner at Winchester College and went then to Exeter College, Oxford, where he graduated first class with a Bachelor of Arts in 1932, winning an honorary scholarship in modern history.
Roskill's oldest sibling is Sir Ashton Roskill QC; another older brother, Stephen, served in the Royal Navy and was an historian. He studied at Winchester College as an exhibitioner before attending Exeter College, Oxford, where he graduated first class with a Bachelor of Arts degree in modern history in 1932.