Masters "grew up in a prefab on the Old Kent Road", Southwark to a "hunchback" mother with a weak chest and an illegitimate "no hoper" father. During his adolescence, after he asked to interview television personality Gilbert Harding for the school magazine he had launched, Harding became close to him, and functioned as a mentor. Masters was apparently quite unfazed when Harding asked to watch him bathe.
The family having moved to Wales in a vain hope of improving his mother's health, Masters read French Literature and Philosophy at University of Wales, Cardiff where he gained a first in 1961. Briefly a teacher in France (as part of his degree), he worked for a time as a travel guide "organising educational tours for American students" and then wrote books on French writers such as Molière (1970) and Camus, among others, without any pretence at them having any real originality. The publisher Anthony Blond interested him in a book on the public's dreams about the Royal Family, which was the first of several books by Masters on the British aristocracy.
Masters is best known for his books about serial killers, written with the co-operation of the subjects or their families. Masters corresponded with Dennis Nilsen from shortly after his arrest in February 1983, and met him in prison without having "felt the slightest unease"  during their time together. His book contains writings by Nilsen, and Masters considers various theories which attempt to explain Nilsen's actions. Masters reaches no final conclusion on "the essential unknowability of the human mind", but Nilsen is "not a stranger amongst us" rather "an extreme instance of human possibility". Masters was accused of being overly sympathetic to Nilsen at the time his book first appeared, a view he rejects in his memoir.
Following the book on Nilsen, Masters wrote The Shrine of Jeffrey Dahmer and She Must Have Known: The Trial of Rosemary West.
- Sartre, a study (1970)
- A Student's Guide to Saint-Exupéry (1970)
- A Student's Guide to Rabelais (1971)
- Dreams About HM the Queen and Other Members of the Royal Family (1973)
- Wynyard Hall and the Londonderry Family (1974)
- Camus A study (1974)
- The Dukes: Origin, Ennoblement and History of 26 Families (1980)
- Georgiana (1981)
- Great Hostesses (1982)
- Killing for Company: The Case of Dennis Nilsen (1985)
- The Life of E. F. Benson (1991)
- The Shrine of Jeffrey Dahmer (1993, 2007)
- She Must Have Known: Trial of Rosemary West (1996)
- The Evil That Men Do (1996)
- Thunder in the Air: Great Actors in Great Roles (2000)
- Getting Personal (autobiography 2002)
- Barber, Lynn (24 August 2002). "The life of Brian". The Observer. ISSN 0261-3077. Retrieved 5 December 2016.
- Gabriele Annan "The beautiful and the damned", - Review of Master's memoir Getting Personal in The Spectator, 31 August 2002. Retrieved on 9 June 2008.
- Quoted by Mark Bostridge "Books: Memoir: Dennis Nilsen's grisly potboiler", - review of Master's memoir Getting Personal in the Independent on Sunday, 25 August 2002, as reproduced on the Find Articles website. Retrieved on 9 June 2008.
- cited by Michiko Kakutani in "Books of The Times; The Portrait of a British Serial Killer", - Review of Killing for Company, New York Times, 26 November 1993. Retrieved on 9 June 2008.
- cited by Ralph Solenko Psychiatry and Criminal Culpability, 1995, John Wiley, p5.