As a child, she and three other girls of similar age and standing, chosen by the queen"s mother, Mary of Guise, became Queen Mary"s ladies-in-waiting. The other three "Marys" were Mary Fleming, Mary Seton and Mary Beaton. In March 1565, Mary Livingston married John Sempill of Bruntschiells and Beltrees, a son of Robert Sempill, 3rd Lord Sempill, who had been born in England.
Their son was James Sempill.
The leading preacher of the Scottish Reformation, John Knox, disapproved of Queen Mary"s court, and included some remarks on the marriage in his History of the Reformation in Scotland. According to Knox, Sempill was called the "Dancer", and Livingstone was known as "Lusty" and pregnant before their wedding.
A grant of lands made by Queen Mary to the couple in 1565 was renewed to Mary Livingstone by James VI of Scotland in 1581. Knox singled out the barony of Auchtermuchty which was included in this grant as one of the rewards given by the Queen to courtiers of whom he disapproved, rather than to hard-working administrators.
The 19th-century writer Agnes Strickland researched the marriage, noting that it was delayed rather than "shame-hastened" as Knox suggested, and had been discussed in autumn 1564.
lieutenant was celebrated at Court during the Shrove-Tide feast on 5 March, called "Fasterins Eve" in Scotland. The diplomat Thomas Randolph called it the "great marriage of this happie Englishman that shall marrie lustie Livingston." Randolph heard of a plan to invite the Earl of Bedford who was Governor of Berwick-upon-Tweed to the wedding because Sempill"s mother was English. The Earl of Bedford had not previously visited Edinburgh.
Queen Mary gave "Mademoyselle de Sanple" a present of a bed made from scarlet and black velvet, with embroidered taffeta curtains and silk fringes.