After A.L. Goodhart's resignation in 1952, Hart was selected to replace him as the Chair of Jurisprudence at Oxford. From 1952 to 1968 Hart served as a professor of jurisprudence and an Oxford University Fellow. He was a visiting professor at Harvard University from 1956 to 1957 and at the University of California at Los Angeles from 1961 to 1962. From 1959 to 1960 he served as the president of the Aristotelian Society. In 1968 he resigned as the Chair of Jurisprudence and spent the next four years as a Nuffield Foundation Senior Research Fellow at University College, Oxford. In 1972 he was named Principal of Brasenose College, Oxford, where he remained until his retirement in 1978. He was a delegate of Oxford University Press beginning in 1960, a member of the Monopolies Commission from 1967 to 1973, and chairperson of the Oxford University Enquiry into Relations with Junior Members in 1969.

As a legal philosopher, Hart drew heavily on the tradition of legal positivism, especially the works of Jeremy Bentham (1748-1832) and John Austin (1790-1859), as well as their follower John Stuart Mills (1806-1873). Legal positivists based their theories on three basic tenets. First, they argued that what the law is and what the law should be are two separate questions; therefore, there is no connection between legality and morality. A law can be legitimized by a society but also be immoral, and an evil regime can still institute a system of laws. Second, they held that the analysis of legal concepts, such as a legal system, rules, and rights, is an important endeavor. Third, they argued that laws are commands issued by a sovereign whom the public obeys out of habit.

Hart concurred on the first two points. However, he disagreed that the foundation of a legal system rests in the power of a sovereign to command. Laws are not obeyed because of the coercive demands of the government, he argued; rather, obligation stems from the social rules of a given society.

In The Philosophy of Law: An Introduction to Jurisprudence, Jeffrie G. Murphy and Jules L. Coleman refer to Hart's The Concept of Law (1961), as being "universally regarded as the most significant contribution to legal philosophy of [the 20th] century. … Hart gives the theory of legal positivism the most systematic and powerful statement it has ever received and is ever likely to receive." In The Concept of Law, Hart argued that Austin laid a foundation for an excellent theory of law by delineating between laws and morals but erred in viewing law as a weapon of a large bully who demands compliance by force.

Hart argued that a legal system is not a compilation of individual laws, but rather a union of primary and secondary rules. Primary rules impose an obligation: what a citizen can or cannot do. Secondary laws define specifics of the primary rules. To exemplify his theory, Hart asked his readers to imagine a pre-legal society, that is, one that lacked laws, and then imagine the types of problems that might plague such a society. To make changes to eliminate these pathologies would be to make laws. The legal system is then, simply defined, those changes prescribed by a society to cure social problems.

Hart noted that even a pre-legal society would follow social norms. Even if there were no specific law against it, walking around naked in most societies would be considered outside the range of acceptable social behavior. Hart called these primary rules: rules directed to all individuals in a given society that impose obligations. Unlike Austin, who suggested that such rules are followed in order to avoid punishment, Hart argued that a primary rule imposes obligation because it sets a standard for criticism or justification within the society.

A society formed around primary rules alone, Hart acknowledged, would suffer from difficulties. First, uncertainty would arise concerning what the rules are, how rules are applied, and what to do if rules conflict. Second, primary rules are static and do not change as the social, economic, and political environment changes. Third, primary rules alone are inefficient because there is no systematically prescribed recourse for when there is conflict over the rules or the rules are broken. In response to these apparent difficulties, Hart suggested the addition of secondary rules, or rules about rules, as he noted in The Concept of Law: "They specify the ways in which the primary rules may be conclusively ascertained, introduced, eliminated, varied, and the fact of their violation conclusively determined." According to Hart, three basic secondary rules exist: rules of recognition, rules of change, and rules of adjudication. They are laws that establish the authoritative structure by which primary laws are defined and enacted.

Hart maintained that a legal system must be approached from an internal viewpoint. As a participant within the system, an observer brings an assumption that the law ought to be obeyed. In The Legal Philosophy of H.L.A. Hart, Michael Martin discusses Hart's theory of internalization: "Social actors can view their own behavior in different ways: they can accept the rules of a system and use them to guide their actions and to evaluate the actions of others or they can follow the rules without accepting them. Hart maintains that to have a legal system at least the officials of the system must take the internal point of view: they must accept the rules of the system and evaluate others' actions in terms of them."

The Concept of Law became the yardstick by which both Hart's supporters and detractors were measured. As McCormick noted, "It is a work of international eminence which even its strongest critics have acknowledged as a masterpiece worth at least the compliment of careful refutation."

The Concept of Law was not Hart's only important book. In 1959 he published Causation in the Law, co-authored by A.M. Honore, which examined questions of causality in matters of civil and criminal liability. Hart and Honore argued for a definition of cause that employed common, everyday language, which is fundamental to basic understanding of what is fair and just in punishment or compensation. In Law, Liberty, and Morality, published in 1963, Hart set forth an argument against legal moralism. He argued that laws and morals are not related by necessity. In 1965 he published The Morality of Criminal Law, in which he laid out a defense of the limits of law in regulating moral behavior. Punishment and Responsibility: Essays in the Philosophy of Law followed in 1968. In it, Hart offered a complex theory of punishment that combines elements of both retribution and utility.

Hart edited three books on Bentham: Jeremy Bentham, Of Laws in General (1970), Bentham, An Introduction to the Principles of Morals and Legislation (1970), and Bentham, Comment on the Commentaries [and] A Fragment on Government (1977). Hart published two more works after his retirement: Essays on Bentham: Studies in Jurisprudence and Political Theory (1982) and Essays on Jurisprudence and Philosophy (1983). Over the course of his distinguished career, Hart wrote numerous published essays. The most influential were "Definition and Theory in Jurisprudence" (inaugural lecture, Oxford, 1953), "Are There Any Natural Rights?" (1967), and "Positivism and the Separation of Law and Morals" (1958).