She married Arthur Forbes of Reres or Rires Castle in Kilconquhar, Fife, and was then called "Lady Reres". The place-name was sometimes written "Reresse". On 22 December 1558 Mary of Guise paid her £300. In September 1566 she was part of the household of James VI of Scotland at Stirling Castle, provided with a bed with curtains made from blue wool cloth called "ostage". In December 1566 Mary bought her a gown of black velvet with black satin sleeves and skirt front. Margaret Beaton, her sister Janet Beaton, and Jean Hepburn were said to Mary's attendants at Dunbar Castle, after Bothwell had abducted her, before the battle of Carberry Hill. In April 1570 Forbes was involved in the murder of Mr John Wood, secretary of Mary's half-brother Regent Moray. She featured in stories about Mary, Queen of Scots written as propaganda by the queen's enemies, which overshadow the few facts known about her role at court. George Buchanan wrote that she was old and fat in the 1560s, "a woman very heavy both by unweildy age and massy substance", and had been a mistress of James Hepburn, 4th Earl of Bothwell. He tells a story of Mary and Margaret Carwood helping Lady Reres with her girdle or belt to climb down a garden wall in Edinburgh's Cowgate in order to bring the Earl of Bothwell to a secret liaison with the queen. This incident or story was also described with more detail at the York Conference in 1568 and recorded in a document called the "Book of Articles"; "In September 1566 she being in Mr John Balfour's house in the Canongate and the Exchequer House, Bothwell resorted to her night and day. Her behaviour in the said Exchequer House shows how at that time she abused her body with him, he resorting through Mr David Chalmer's lodging where she lay, conveyed to her by the Lady Rires means, which she herself confessed to diverse people. And especially Bothwell breaking the apppointed tryst at one time, and she impatient of his tarry and delay, sent Lady Rires to his chamber for him, which lady passing over the dyke at the nearest, took him out of his bed from his wife and brought him to the queen." A summary of arguments against Mary produced in 1568 by William Cecil, 1st Baron Burghley, called the "Contra Reginam Scotorum" asserted that Bothwell obtained his divorce from Jean Gordon because of his "frequent advouterie" – adulteries, especially with Lady Reres. She was also the "principal instrument" between the queen and Bothwell. One of the Casket Letters mentions a conversation at Callendar House with Lord Livingston,and that Lord Darnley mentioned her. Darnley's comment, that "she should serve her well", was mentioned in Thomas Crawford's declaration. Richard Bannatyne, the secretary of John Knox, wrote that Mary had an easier childbirth because of the sorcery of Margaret Fleming, Countess of Atholl who transferred the queen's pains to Lady Reres, who was also in childbirth in Edinburgh Castle at the same time. Lady Reres on this occasion was in more pain than during her other pregnancies. Bannatyne says that John Knox heard this story from one Andrew Lundie, who may have been a relation of Forbes of Reres. In 1573, during the "lang siege" of Edinburgh Castle in the Marian Civil War, Lady Rires, her niece Lady Jean Stewart, and Mary Fleming, William Maitland's wife, stayed in the castle.
Family
The children of Margaret Beaton and Arthur Forbes included:
