Mary was born at Dalkeith Castle, Midlothian, to Francis Scott, 2nd Earl of Buccleuch and his wife, Lady Margaret Leslie, daughter of John Leslie, 6th Earl of Rothes.
In 1651, her father died, making four-year-old Mary, who was the Earl's eldest daughter, the suo jure countess of Buccleuch.
On 9 February 1659, aged only eleven years old, she married Walter Scott of Highchester, who was at that time fourteen.
It created a lot of disapproval and the court ruled that the couple should be separated until Mary reached the age of twelve.
However, she fell ill and died two years after their reunion, aged thirteen, in 1661, and her titles passed to her sister, Anne.