In 1952, Hart was elected
Professor of Jurisprudence at Oxford and was a fellow at
University College, Oxford, from 1952 to 1973. It was in the summer of that year that he began writing his most famous book,
The Concept of Law, though it was not published until 1961. In the interim, he published another major work,
Causation in the Law (with Tony Honoré) (1959). He was president of the
Aristotelian Society from 1959 to 1960. He gave the 1962 Master-Mind Lecture. Hart married
Jenifer Fischer Williams, a civil servant, later a senior civil servant, in the Home Office and, still later, Oxford historian at
St Anne's College (specialising in the history of the police). Jenifer Hart was, for some years in the mid-1930s and fading out totally by decade's end, a 'sleeper' member of the
Communist Party of Great Britain. Three decades later she was interviewed by
Peter Wright as having been in a position to have passed information to the Soviets, and to Wright, MI5's official spy hunter, she explained her situation; Wright took no action. In fact her work as civil servant was in fields such as family policy and so would have been of no interest to the Soviets. The person who recruited her,
Bernard Floud, interviewed by Wright shortly after, maintained that he was unable to remember ever having done so. Nor was her husband in a position to convey to her information of use, despite vague newspaper suggestions, given the sharp separation of his work from that of foreign affairs and its focus on German spies and British turncoats rather than on matters related to the Soviet ally. In fact, Hart was anticommunist. The marriage contained "incompatible personalities", though it lasted right to the end of their lives and gave joy to both at times. Hart did joke with his daughter at one point, however, that "[t]he trouble with this marriage is that one of us doesn't like sex and the other doesn't like food", and according to Hart's biographer, LSE law professor Nicola Lacey, Hart was by his own account, a "suppressed homosexual". Jenifer Hart was believed by her contemporaries to have had an affair of long duration with
Isaiah Berlin, a close friend of Hart's. In 1998, Jenifer Hart published
Ask Me No More: An Autobiography. The Harts had four children, including, late in life, a son who was disabled, the umbilical cord wrapped around his neck having deprived his brain of oxygen. Hart's granddaughter
Mojo Mathers became
New Zealand's first deaf Member of Parliament in 2011.
Karen Armstrong, an author who wrote about religion, referenced the Harts' household. She lodged with them for a time to help take care of their disabled son. The description appears in her book,
The Spiral Staircase. Hart retired from the Chair of Jurisprudence in 1969 and was succeeded by
Ronald Dworkin. He subsequently became principal of
Brasenose College, Oxford. Hart died in Oxford on 19 December 1992, aged 85. He is buried there in
Wolvercote Cemetery, which also contains Isaiah Berlin's grave. in Oxford ==Hart's students==