Regan graduated from Thiel College in 1960, receiving his M.A. in 1962 and his Ph.D. in 1966 from the University of Virginia. He taught philosophy at North Carolina State University from 1967 until 2001.

In The Case for Animal Rights, Regan argues that non-human animals bear moral rights. His philosophy aligns broadly within the tradition of Immanuel Kant, though he rejects Kant's idea that respect is due only to rational beings. Regan points out that we routinely ascribe inherent value, and thus the right to be treated with respect, to humans who are not rational, including infants and the severely mentally impaired.

The crucial attribute that all humans have in common, he argues, is not rationality, but the fact that each of us has a life that matters to us; in other words, what happens to us matters to us, regardless of whether it matters to anyone else. In Regan's terminology, we each experience being the "subject-of-a-life." If this is the true basis for ascribing inherent value to individuals, to be consistent we must ascribe inherent value, and hence moral rights, to all subjects-of-a-life, whether human or non-human. The basic right that all who possess inherent value have, he argues, is the right never to be treated merely as a means to the ends of others.

In Regan's view, not to be used as a means entails the right to be treated with respect, which includes the right not to be harmed. This right, however, is not absolute, as, there are times when to respect someone’s right not to be harmed, another’s right not to be harmed must be overridden. His philosophy employs principles such as the miniride principle (a.k.a. minimize overriding) and the worse-off principle to deal with these situations. The miniride principle is that when faced with overriding the rights of many innocent beings versus the rights of few innocent beings—when each individual involved would be equally harmed—we should override the rights of the few. The worse-off principle states that, when individuals involved are not harmed in a comparable way given a certain course of action, we should mitigate the situation of those who would be worse-off. Thus, if the harm of a few innocent beings is greater than the harm to many innocent beings, the right action is to override the rights of the many. As this relates to animal rights, Regan asserts the harm in death of an animal is not tantamount to the harm in death of a normal, healthy human. This is supposedly because the ending of an animal life entails the loss of fewer opportunities when compared to the loss of a normal, healthy human’s. Note that arguably this does not make Regan vulnerable to the charge of speciesism, as the evaluation of harm is based on a criterion of opportunity, not in mere species membership. According to Regan, there would be more harm in the death of a normal, healthy dog than there would be in the death of a person who was irreversibly comatose, as the dog would have more opportunities for satisfaction than the irreversibly comatose human.

Supporters argue that Regan's argument for animal rights does not rely on a radical new theory of ethics, but that it follows from a consistent application of moral principles and insights that many of us already hold with respect to the ethical treatment of human beings. However, others criticize the lack of certainty with which Regan's "inherent value" or "subject-of-a-life" status can be determined, and note that the sufficient conditions he lists — for example, having sense-perceptions, beliefs, desires, motive, and memory — in effect reduce to "similarity to humans".