Mary Hays was born in the Southwark district of London, one of several children of John Hays and his wife Elizabeth. In 1779, she fell in love with and became engaged to John Eccles, who died the following year, shortly before the marriage was to take place. The tragedy may have spurred Mary's writing career and her subsequent immersion in radical intellectual circles. Mary wrote political pamphlets such as Cursory Remarks on An Enquiry into the Expediency and Propriety of Public or Social Worship, using the pseudonym Eusebia (1791) and articles for the Analytical Review, a liberal magazine. Her most notorious and popular work was Memoirs of Emma Courtney (1796). It was one of the most articulate and detailed expressions of the yearnings and frustrations of a woman in late 18th-century English society. Written in the turbulent years after the start of the French revolution, Memoirs of Emma Courtney questioned contemporary marital arrangements and explored the links between sexuality, desire, and economic and social freedom. In it, Mary Hays urged reforms in the laws of society that "have enslaved, enervated, and degraded woman." Her other works included Letters and Essays, Moral and Miscellaneous (1793) and Appeal to the Men of Great Britain on behalf of the Women (1798), which was published anonymously. She also published the six-volume Female Biography (1802). Mary Hays was a friend and passionate disciple of the feminist Mary Wollstonecraft, and is credited with introducing her to William Godwin, whom she married in 1797. When Mary Wollstonecraft was dying due to complications of childbirth, Mary Hays helped to nurse her. She also wrote Mary Wollstonecraft's obituary. Mary Hays was satirized by some contemporary writers, notably as Bridgetina Botherim in Elizabeth Hamilton's novel Memoirs of Modern Philosophers (1800) and as Lady Gertrude Sinclair by Charles Lloyd in his Edmund Oliver (1798).