He is a retired general practitioner who published books and articles on medical subjects, but who is mainly known for his extensive contributions to Anglo-Saxon history.
Hart was born on 16 May 1923 at his grandparents' home, 12 Benson Avenue, East Ham, an eastern suburb of London.
[1] After Hart had qualified as a physician, he turned to the serious study of history and gained a master's degree at Leicester University under the supervision of Herbert Finberg.
He published several studies of Anglo-Saxon charters which, in the view of Simon Keynes and Alfred Smyth, secured him a reputation on a par with leading professional historians.
He went on to publish The Danelaw, described by Keynes and Smyth as "a monumental work running to 700 pages of immensely detailed scholarship".