Unger attended Swarthmore College at the same time as David Lewis, earning a B.A. in philosophy in 1962, and Oxford University, where he studied under A. J. Ayer and earned a doctorate in 1966.

Unger has written a defense of profound philosophical skepticism. In Ignorance (1975), he argues that nobody knows anything and even that nobody is reasonable or justified in believing anything.

In Philosophical Relativity (1984), he argues that many philosophical questions cannot be definitively answered.

In the field of applied ethics, his best-known work is Living High and Letting Die (1996). In this text, Unger argues that the citizens of first-world countries have a moral duty to make large donations to life-saving charities (such as Oxfam and UNICEF), and that once they have given all of their own money and possessions, beyond what is needed to survive, they should give what belongs to others, even if having to beg, borrow, or steal in the process.

In "The Mental Problems of the Many" (2002), he argues for substantial interactionist dualism on questions of mind and matter: that each of us is an immaterial soul. The argument is extended and fortified in his 2006 book All the Power in the World.

In Empty Ideas (2014), he argues that analytic philosophy has delivered no substantial results as to how things are with concrete reality.