Mary Berenson (born Mary Whitall Smith; 1864 in Pennsylvania – 1945 in Italy) was an art historian, now thought to have had a large hand in some of the writings of her second husband, Bernard Berenson.
Her father was Robert Pearsall Smith, her mother Hannah Whitall Smith (born Hannah Tatum Whitall). She studied at the Harvard Annex in 1884-1885. Here Mary met the Irish barrister Benjamin "Frank" Conn Costelloe, whom she married in 1885, having converted to Catholicism. This marriage was the occasion for the whole family, including her brother Logan Pearsall Smith and sister Alys Pearsall Smith to move to England in 1888. However, already in 1892 the couple separated, though Frank would not agree to divorce.
Mary had two daughters with Frank Costelloe; Ray Strachey and Karin Stephen. Through them Mary was related by marriage to the Bloomsbury Group of English artists and literary figures: Karin married Adrian Stephen who was Virginia Woolf's brother, and Ray married Oliver Strachey who was Lytton Strachey’s brother 
In 1888 in London she made the acquaintance of Bernard Berenson. She became an authority on art history and she took up with Berenson in Italy. Berenson developed a reputation as an art expert and it is believed Bernard's work were substantially helped by Mary. Their book "The Venetian Painters of the Renaissance" was published in 1894 under Berenson's name and it is said that it was Mary's mother who asked that Mary not take credit for her work.
She and Berenson eventually married in 1900 after her first husband died, although they both had affairs and Mary believed this was because they liked to hurt each other.
Her US lecture tours were instrumental in developing an interest in Italian Renaissance art among wealthy American collectors during the first decade of the twentieth century.
Subsequently, Berenson brought together a social circle at Villa I Tatti, the Berenson home, and developed its gardens. She hosted some of the most celebrated personalities of the period, including Edith Wharton, Gertrude Stein, Gabriele D'Annunzio, John Maynard Keynes, and Isabella Gardner.
By 1927 Mary tired of entertaining, and left the duty of hosting to the couple's librarian Elizabeth Mariano. Mariano was one of Bernard's lovers, and Mary would much later write to give permission for Mariano to marry Bernard. In later life she was plagued by illness and by 1935 she was largely an invalid. In 1940 she lost her eldest daughter to surgery complications, and she was left by her husband under Fascist regulations, (as he was a Jew), in the care of Mariano's sister. Mary remained in I Tatti until she died in 1945.
- "Dictionary of Art Historians - Mary Berenson". arthistorians.info. Retrieved 2016-03-28.
- Tiffany L. Johnston (2012). "Mary Whitall Smith at the Harvard Annex". Berenson and Harvard. The President and Fellows of Harvard College. Retrieved June 4, 2014.
- The Strachey Papers at the Archives in London and the M25 Area
- Palmer, Alan (1987). Who's Who in Bloomsbury. New York: St. Martin's Press. pp. 17–18.
- Booton, Diane E. (2011-06-16). "Mary Costelloe Berenson". Harvard Magazine. Retrieved 2019-09-24.
- Johnston, Tiffany (2015). "Mary Berenson and the Cultivation of American Collectors," in A Market for Merchant Princes: Collecting Italian Renaissance Paintings in America. University Park, PA: Penn State University Press. p. 72. ISBN 978-0-271-06471-0.
- The garden of Villa I Tatti: some historical notes in The Harvard university Center for Italian Renaissance Studies Archived 2007-09-27 at the Wayback Machine
- Barbara Strachey and Jayne Samuels (1983), Mary Berenson: a Self Portrait from her Letters & Diaries
- Berenson, Mary (1864–1944) at encyclopedia.com