She was the daughter of Count Nikolay Moritsevitch O'Rourke, a Russian naval officer of Irish ancestry and his second wife Ekaterina Seletska - a noblewoman of Cossack origin.
Her 1910 trial in Venice and subsequent conviction attracted media attention from both sides of the Atlantic and became the subject of various books (Annie Chartres Vivanti, Hans Habe, etc.
The Countess Tarnowska, as she was commonly called, was arrested that same year in Vienna and transferred to La Giudecca penitentiary in Venice, where the trial was to be held.
The trial, locally called "the Russian affair" (il caso russo), began on 14 March 1910 and ended on 20 May of the same year, with the conviction of both defendants.
Maria Tarnowska was found guilty, but was sentenced to serve a relatively mild term of only eight years in prison, thanks to an ingenious defence (it was one of the first to include Freudian analysis of the defendant's personality and motives) – and, possibly, due to the leniency of the presiding judge.