Princess Mary of Romania was born in Gotha, Thuringia in Germany, and was always known by the name Mignon. Her father was King Ferdinand of Romania, who reluctantly agreed to accept paternity to avoid scandal, and her mother, known universally as Queen Marie of Romania, was born Princess Mary of Edinburgh, the daughter of Prince Alfred, Duke of Edinburgh, a son of Queen Victoria. Princess Mary of Romania's maternal grandfather was Tsar Alexander II of Russia. Her biological paternity is questionable: Marie told Ferdinand that the biological father was Boris Romanov (1877-1943).
She married King Alexander on 8 June 1922.
She became Yugoslav Queen Mother when, following the assassination of King Alexander in Marseilles in 1930 her oldest son became Peter II of Yugoslavia, the last Yugoslav king.