The actress has signed on for the lead role in the Michael Hirst-penned biopic, reports Deadline.
Working Title is currently seeking a director for the period drama.
Mary's life was a mess of intrigue that saw her abdicate her throne after marrying her late husband's supposed murderer.
Seeking refuge with her cousin, Elizabeth I of England, she was imprisoned and later executed. Her son James I later became king of Great Britain and Ireland.
Hanna star Ronan will next appear in Violet & Daisy, Byzantium and The Host.
Watch Saoirse Ronan telling Digital Spy about The Host below: