Mary Berenson, née Smith, was born in Germantown, Pennsylvania to two Quaker preachers, Robert Pearsall Smith and his wife Hannah Whitall. Her brother was the literary essayist Logan Pearsall Smith. She attended Smith College and Harvard Annex, now Radcliffe. In 1885, she converted to the Roman Catholic faith to marry Benjamin Francis Conn "Frank" Costelloe, an Irish barrister and future politician whom she had met at Harvard. The couple went to live in England and had two daughters. She became involved in the women's suffrage movement. She met art historian Bernard Berenson during a visit he made to her home in 1890. She separated from Costelloe in 1892, and turned her interests to art history and Berenson, with whom she traveled in Europe. Under the pseudonym Mary Logan, she wrote journal articles and a long pamphlet, Guide to the Italian Pictures at Hampton Court: with Short Studies of the Artists (1894), which established her as an art authority. She helped Berenson write the book Venetian Painters of the Renaissance, which appeared the same year and made his reputation as an art historian. Costelloe died in 1899 and Mary married Berenson in 1900 in a small chapel on his estate of Villa I Tatti near Florence, Italy. Over the years, she devoted much of her energy to supporting Berenson's work and editing his manuscripts, but continued to publish her own work as magazine articles and books, including A Tentative List of Italian Pictures Worth Seeing (1908), A Modern Pilgrimage (1933) and A Vicarious Trip to the Barbary Coast (1938). The couple entertained lavishly at their villa until the 1929 stock market crash. During World War II, as American expatriates, they were prevented from leaving Italy. She remained at I Tatti while Berenson, who was born Jewish, hid in the home of a friend. She died at the end of 1944 and was buried at I Tatti.