Williams was educated at Chigwell School, Essex, and Balliol College, Oxford. During the 1950s he served in the Royal Air Force (1951–53) and was a fellow of All Souls College and New College, Oxford. He was appointed Knightbridge Professor of Philosophy at the University of Cambridge in 1967 and Provost of King's College, Cambridge, in 1979. He was Monroe Deutsch Professor of Philosophy at the University of California, Berkeley, from 1988 to 2003 and White's Professor of Moral Philosophy at Oxford from 1990 to 1996.

In 1955 Williams married Shirley Catlin, who, as Shirley Williams, became a prominent political figure in Britain; in 1993 she was created Baroness Williams of Crosby. In 1974 the marriage was dissolved, and Williams married Patricia Skinner. Williams headed or served on a number of public commissions, notably the Committee on Obscenity and Film Censorship (1977–79), and was a director of the English National Opera. He was knighted in 1999.

Williams was trained in classics and wrote memorably about Plato, Aristotle, and Greek moral consciousness, but he was also one of the most prolific and versatile philosophers of his time. His published works include writings on René Descartes (Descartes, René) (1596–1650), Friedrich Nietzsche (Nietzsche, Friedrich) (1844–1900), and Ludwig Wittgenstein (Wittgenstein, Ludwig) (1889–1951) and important papers and books on personal identity, the relation of morality to human motivation, the idea of social and political equality, the nature and value of truth, the significance of death, and the role and limits of objectivity in science, morality, and human life. He did not put forward a systematic philosophical theory; indeed, he was suspicious of systematic theories, particularly in ethics, because, in his view, they failed to be true to the contingency, complexity, and individuality of human life.

Williams was recognized for his brilliance even as an undergraduate. He was trained in philosophy when Oxford was home to the new movement of linguistic analysis, or ordinary language philosophy (ordinary language analysis), led by J.L. Austin (Austin, John Langshaw), but the breadth of his cultural, historical, and political interests kept him from becoming an adherent of that school. He met its standards of clarity of expression and rigour in argument, but his aims in philosophy went far beyond conceptual analysis (see analytic philosophy). He regarded philosophy as an effort to achieve a deeper understanding of human life and the human point of view in its multiple dimensions. For the same reasons, he also resisted the tendency to regard scientific knowledge as the model of understanding to which philosophy should aspire at a more abstract level—a tendency that was strengthened during his lifetime by the growing influence of the American philosopher W.V.O. Quine (Quine, Willard Van Orman) and by a shift in the centre of gravity of English-language philosophy from Britain to the United States. Williams held that physical science could aspire to an objectivity and universality that did not make sense for humanistic subjects, and his greatest influence came from his challenge to the ambition of universality and objectivity in ethics, especially as expressed in Utilitarianism but also in the tradition established by Immanuel Kant (Kant, Immanuel).

In his book Descartes: The Project of Pure Inquiry (1978), Williams gave a compelling description of the ideal of objectivity in science, which he called the “absolute conception” of reality. According to this conception, different human perspectives on and representations of the world are the product of interaction between human beings, as constituents of the world, and the world itself as an independently existing reality. Humans cannot apprehend the world except by some form of perception or representation; yet they can recognize, and to some extent identify and try to compensate for, the distortions or limitations that their own point of view and their relation to the rest of reality introduce. The aim of objectivity in science is by this method to approach as closely as possible to the absolute conception—a conception of what is there “anyway,” independent of the human point of view. Historical self-consciousness about the contingent elements in this process is compatible with the idea of a single truth toward which humans are trying to make progress.

Williams was cognizant of the doubts that exist regarding whether this ideal is intelligible, let alone attainable, in light of the fact that human thinking must start from some particular historical moment and must use the contingent biological faculties and cultural tools that happen to be at hand. But whatever may be the difficulties in pursuing this ideal, he believed that it makes sense as an ambition. If there is a way things are anyway, then it makes sense to want to know what that way is and to explain the nature of human perceptions in terms of it.

Some philosophers, in the tradition of David Hume (Hume, David) (1711–76), have denied that there can be objective truth in ethics on the ground that this would have to mean, very implausibly, that moral propositions are true because they represent moral entities or structures that are part of the furniture of the world—moral realities with which humans have some kind of causal interaction, as they do with the physical objects of scientific knowledge. Williams was also doubtful about objectivity in ethics, but his criticism does not depend on this false analogy with science and is more interesting.

Moral judgments, according to Williams, are about what people should do and how they should live; they do not at all purport to represent how things are in the outside world. So, if there is any objectivity in moral judgments, it would have to be sought in a different analogy with scientific objectivity. Objective truth in ethics would have to consist not in ethical entities or properties added to the absolute conception of the external world but in the objective validity of the reasoning that supports certain practical, rather than descriptive, judgments about what people should do and how they should live. The analogy with scientific objectivity would reside in the fact that the way to arrive at such objective and universally valid truth would be to detect and correct for the biases and distortions introduced into one's practical judgment by contingencies of one's personal or parochial perspective. The more distortions one could correct, the closer one would get to the truth.

But it is just this aim, central to the idea of moral objectivity, that Williams thinks is fundamentally misguided. Williams finds something bizarre about the theoretical ambition of discovering a standard for practical judgment that escapes the perspectival peculiarities of the individual point of view. This applies to any ethical theory with a strong basis in impartiality or with a claim to universal validity. Williams's basic point is that, in the practical domain, the ambition of transcending one's own point of view is absurd. If taken seriously, it is likely to be profoundly self-deceived in its application. To Williams, the ambition is akin to that of the person who tries to eliminate from his life all traces of the fact that it is his.

Williams developed this objection through his general view that practical reasons must be “internal” rather than “external”: that is, reasons for action must derive from motives that a person already has; they cannot create new motives by themselves, through the force of reason alone. He also defended a limited form of ethical relativism. He believed that, while there can be ethical truth, it is local and historically contingent and based on reasons deriving from people's actual motives and practices, which are not timeless or universal. Consequently, moral judgments cannot be applied to cultures too far removed in time and character from the culture in which they originate.

These arguments appear in Ethics and the Limits of Philosophy (1985), "A Critique of Utilitarianism" (1973; in Utilitarianism: For and Against), and some of the essays reprinted in Moral Luck (1981) and Making Sense of Humanity (1995). The debate provoked by Williams's claim that impersonal moral standards undermine the integrity of personal projects and personal relations, which give life its very meaning, was an important part of the moral and political philosophy of the later 20th century. Even philosophers who did not accept Williams's conclusions were in most cases led to recognize the importance of accommodating the personal point of view as a factor in moral theory.

Williams also raised and explored the deep question of whether a person's moral status is immune to “luck,” or purely contingent circumstances, as Kant had argued (for Kant, the moral status of an individual depends only on the quality of his will). Williams invented the concept of “moral luck” and offered strong reasons to think that people are morally vulnerable to contingencies beyond their control, a conception he found exemplified in Greek tragedy. Oedipus, for example, is not relieved of guilt for killing his father and marrying his mother by the fact that he did not know at the time that that was what he was doing. Williams's remarkable philosophical-literary-historical work, Shame and Necessity (1993), presented these ideas in a rich study of Greek ethical thought.

Williams came to the conclusion that, instead of following the model of natural science, the project of understanding human nature should rely on history, which provides some distance from the perspective of the present without leaving the fullness of the human perspective behind. During the later part of his career, this viewpoint coincided with his admiration for Nietzsche, whose genealogical method was an example of historical self-exploration. In Williams's last book, Truth and Truthfulness (2002), he applied these ideas to the importance of truth in the theoretical and practical spheres as well as in political and personal relations.

Williams's major published works, in addition to those mentioned above, include Morality: An Introduction to Ethics (1972); Problems of the Self: Philosophical Papers (1973); In the Beginning Was the Deed: Realism and Moralism in Political Argument (2005); The Sense of the Past: Essays in the History of Philosophy (2005); Philosophy as a Humanistic Discipline (2005); and On Opera (2006).