Best known as the author of On Liberty, John Stuart Mill remains a canonical figure in liberalism today. Yet according to his autobiography, by the mid-1840s he placed himself "under the general designation of Socialist." Taking this self-description seriously, John Stuart Mill, Socialist reinterprets Mill's work in its light. Helen McCabe explores the nineteenth-century political economist's core commitments to egalitarianism, social justice, social harmony, and a socialist utopia of cooperation, fairness, and human flourishing. Uncovering Mill's changing relationship with the radicalism of his youth and his excitement about the revolutionary events of 1848, McCabe argues that he saw liberal reforms as solutions to contemporary problems, while socialism was the path to a better future. In so doing, she casts new light on his political theory, including his theory of social progress; his support for democracy; his feminism; his concept of utility; his understanding of individuality; and his account of "the permanent interests of man as a progressive being," which is so central to his famous harm principle. As we look to rebuild the world in the wake of financial crises, climate change, and a global pandemic, John Stuart Mill, Socialist offers a radical rereading of the philosopher and a fresh perspective on contemporary meanings of socialism.