Philosopher, essayist, and intellectual partner of John Stuart Mill, whose work she shaped profoundly for more than two decades before their marriage in 1851 and the seven years of formal collaboration her health permitted before her death in 1858. The precise allocation of credit between them is one of the hardy perennials of Mill scholarship; Mill himself insisted, publicly and privately, that her contribution to On Liberty, The Principles of Political Economy, and especially the ideas in The Subjection of Women was foundational — not ornamental.
Her own signed writing is sparse but sharp. The Enfranchisement of Women (1851), published under her name in the Westminster Review, is the clearest statement of her position: the case for women's political, legal, and educational equality rests on the same liberal principles — consent, development, the value of diverse experiments in living — that the tradition has always claimed to honor but systematically denied to half its members. The essay anticipates, in compressed form, the arguments her husband would develop at length in 1869.
She also pressed Mill leftward on economic questions, encouraging the cautious socialism of his later editions. Victorian convention and subsequent sexist dismissal long buried her; serious scholarship since the 1990s has steadily restored her to view.