Mary (Molly) Josepha Warre-Cornish was born in August 1882. Her father, Francis Warre Warre-Cornish (1839–1916) was vice-provost of Eton College. Her mother, Blanche Ritchie (1848–1922) was a novelist.
Mary married Desmond MacCarthy on 29 August 1906. Over the next few years she had two sons, Michael and Dermod, and a daughter, Rachel. She became a member of the Bloomsbury Group, a group of men and women that discussed literary and artistic issues. Members included Vanessa Bell, Virginia Woolf, Clive Bell, John Maynard Keynes, E. M. Forster, Leonard Woolf, Lytton Strachey, David Garnett, Duncan Grant, Arthur Waley and Saxon Sydney-Turner.
Mary MacCarthy died of heart failure on 28th December 1953 at Garrick's Villa, Twickenham.