Her mother, Mary Britneva, was British by birth, a daughter of Charles Herbert Bucknall, business partner in St Petersburg of the French wholesale gem dealers Leo and Georges Sachs.
[2] In the summer of 1922, when Britneva was just thirteen months old, her mother left Russia and emigrated to England, taking with her Maria and her brother Vladimir.
[2] In 1939, when registered at the outset of the Second World War, her mother was living in Fulham and stated her occupation as “writer and translator” and her date of birth as 3 April 1894.
[2] As a child, Britneva studied ballet with Tamara Karsavina and was known as "the little grasshopper" for her ability to jump high, but later she could not pursue a career as a dancer because she was too small[3] or because of foot trouble and, she said, overly large breasts.
[15] In an article published in The New Yorker soon after her death, John Lahr wrote that he believed Britneva reminded Williams of his mother.
[2] In 1955, Williams said after Britneva’s opening night performance as Blanche in A Streetcar Named Desire, in a production in Florida, "I thought I had written a good play till I saw her in it.
"[16] Renowned as a hostess, surrounded by close friends John Gielgud and Gore Vidal, Franco Zeffirelli, Leslie Caron and Tennessee Williams - her greatest talent may have been for friendship.
[8] As a hostess Maria lionised performing artists, and her lunch and dinner parties would typically include figures such as Claudio Abbado, Darcy Bussell, Lord Goodman, Ishmail Merchant and James Ivory, Leslie Caron and Placido Domingo.
In 1964, Britneva’s mother died at the St George’s Retreat, Burgess Hill, West Sussex, and was buried in Earl's Court, her funeral being conducted by the Russian Orthodox Bishop of Great Britain, Nikodem.
[28] She was increasingly protective of him, going so far as to attempt to push his brother Dakin off a catwalk at the Lyceum Theatre after the Broadway opening of Out Cry in 1973.
[2][19] In 1975, Williams angered Britneva by mentioning her only briefly in his memoirs, in which he referred to her as "an occasional actress" and said she was "afflicted with folie de grandeur".
[2] Williams named Maria St Just as co-trustee (with John Eastman, a celebrity lawyer and the brother of Linda McCartney) of the trust for his lobotomized sister, Rose.
In this role, she fiercely defended his legacy, to an extent that many found excessive, such as involving herself in casting and advising actors, denying scholars access to Williams's papers, demanding the right to vet the manuscript of the authorized biography, and rescinding permission that Williams had granted to Lyle Leverich for such a biography.
[3] In the book, Britneva changes Brooks Atkinson's review in The New York Times of her 1955 performance as Blanche in A Streetcar Named Desire from a pan to a rave.