Gallie grew up in a British boarding school and later published his memoirs of this in the book An English School. He taught at University College, North Staffordshire, where he published (among other things) "Explanations in History and the Genetic Sciences" (Mind, Vol. 64, No. 254 [Apr. 1955], pp. 160–180). His other well-known works include Philosophy and the Historical Understanding, (1964), Philosophers of War and Peace (1978 by Oxford University Press), and Wanted: A Philosophy of International Relations (1979).
His paper on 'Essentially Contested Concepts' (Proceedings of the Aristotelian Society, vol. 56, 1956, pp. 167-198) is a seminal statement in the philosophy of the social sciences. Here, Gallie argued that it is impossible to conclusively define key appraisive concepts such as 'social justice,' 'democracy,' 'Christian life', 'art', 'moral goodness' and 'duty', although it is possible and rational to discuss one's justifications for holding one interpretation over competing ones. Clarification of such concepts involves not the examination of predictive relations (as is the case for most scientific concepts), but rather, consideration of how the concept has been used by different parties throughout its history.
He was president of the Aristotelian Society from 1970 to 1971.