Mary was the twelfth or thirteenth child of Edward I and Eleanor of Castile, and the fourth to survive childhood. She was born at Woodstock on 11 or 12 March 1279, fourteen months after her closest sibling, a girl whose name name is unknown, born in January 1278 and died soon after, and three years younger than Berengaria, born May 1276 and died June 1278 (Edward I and Eleanor lost two daughters in 1278). After Mary's birth, however, Eleanor of Castile had a well-deserved break from her almost yearly pregnancies, and there was a gap of three and a half years to the next child, Elizabeth. (There's a theory, which I don't believe, that Eleanor gave birth to a son in 1280 or 1281.) In the ten years between June 1269 and March 1279, Edward I and Queen Eleanor had eight children, of whom only one (Alfonso) was male.
Mary's grandmother Jeanne de Dammartin, countess of Ponthieu and dowager queen of Castile and Leon, died five days after her birth. In 1285, her other grandmother Eleanor of Provence, widow of Henry III, retired to the priory of Amesbury in Wiltshire, and decided to take two of her granddaughters with her for company. One was Mary; the other was Eleanor of Brittany, daughter of Edward I's sister Beatrice and Duke John II of Brittany, and sister of Arthur, duke of Brittany and John, earl of Richmond. Young Eleanor later became abbess of Fontevrault, the mother house of Amesbury. Unfortunately, Mary had no vocation whatsoever, and her mother Eleanor of Castile was most reluctant for her to become a nun. However, Eleanor of Provence prevailed.
Mary went to live at Amesbury in 1285, and was veiled as a nun in late 1291, aged twelve, in the presence of her father and siblings, including seven-year-old Edward of Caernarfon. For most of her life, Mary spent almost as much time away from Amesbury priory as she spent in it, visiting family, the courts of her father and brother, and going on pilgrimage. This was tolerated by the abbess, because of Mary's high birth - as daughter and sister of kings, she could wield a great deal of influence. Her father granted her an income of £100 a year, later raised to £200.
Mary lived the life of a great lady, not that of a nun. She had private rooms at Amesbury, with a magnificent bed hung with velvet and tapestry, and sheets of linen. On one occasion, her father sent her over two hundred ells of fine cloth via the bishop of Chester, and at another time, two thousand stock-fish. Mary travelled to court with damsels, attendants, and between twenty-four and thirty horses and grooms for them. Her father and brother often paid her gambling debts. She also owned hunting dogs.
Mary had a strong sense of family. In a period of just over three months in 1305, she visited her little half-brothers Thomas and Edmund (sons of Edward I and Marguerite of France, twenty-one and twenty-two years her junior) no fewer than eleven times, staying for up to five days each time. On at least one occasion, her brother Edward of Caernarfon accompanied her. Mary also had the chance to enjoy the company of many of her female relatives at Amesbury. Her niece Joan de Monthermer, Joan of Acre's daughter, and her cousin Isabel, daughter of Henry of Lancaster, were also veiled as nuns at Amesbury (Isabel of Lancaster later became the abbess). In addition, some royal girls not destined to become nuns grew up at Amesbury: Mary's little half-sister Eleanor, who died in 1311 at the age of five, her niece Eleanor de Bohun, daughter of her sister Elizabeth, and her great-niece Joan Gaveston, Joan of Acre's granddaughter. (It's also possible that other royal girls grew up at Amesbury, but evidence is lacking). Mary seems to have been especially close to her niece Elizabeth de Clare, another daughter of Joan of Acre. In late 1316 or early 1317, Elizabeth retired to Amesbury and gave birth to her daughter Isabella de Verdon there. A few weeks later, Mary, Elizabeth and Isabel of Lancaster went on pilgrimage to Canterbury together.
There is much evidence of great affection between Mary and her brother Edward II, five years her junior, and they visited each other often in childhood and adolescence. In 1304, Edward sent Mary a gift of a greyhound - evidently his favourite breed of dog - and in 1307, she sent him a falcon. A nice letter of Edward's survives from 1304, where he apologises to Mary for not sending gifts of several tuns of wine and an organ to her as promised, but the only wine his agents could find to buy was not of sufficiently high quality, and the organ had arrived broken and he was sending it for repair. In 1305, when Edward quarrelled with his father and had his income cut drastically, Mary invited him to stay with her - though unlike her feistier elder sister Joan of Acre, she made sure she got their father's permission first.
After Edward's accession in 1307, Mary continued to visit court often, with her numerous attendants. He often sent her gifts, too; in 1318, he bought expensive Lucca cloth for three people, himself, his current favourite Roger Damory, and Mary. For New Year 1317, he sent her a ring worth ten pounds (and also sent rings to his nieces Margaret and Elizabeth de Clare, his great-niece Joan Gaveston, and his sons Edward and John - rather oddly in the last case, as John was only a few months old).
Queen Isabella corresponded with Mary fairly often, and in 1316, they went on pilgrimage to Canterbury together, offering cloth of gold, saffron and other spices at the shrine. Edward II paid all the expenses of his wife and sister. Edward took a keen interest in Mary's welfare, and paid her expenses when she came to visit him. For example: in January 1313 he ordered the sheriff of Wiltshire to pay her twelve pounds, seven shillings and sixpence "which the king owes her for hay, oats, litter, farriery, and the wages of her grooms" when she stayed with him at Windsor the previous Christmas. He also granted her the manor of Ludgershall (not sure whether that's the Wiltshire one or the Buckinghamshire one) and sent her ten tuns of wine (9540 litres, or about 2510 US gallons) every year.
The only letter I've been able to find from Mary to Edward concerns the election of a new abbess of Amesbury, and begins "To the very high and noble prince, her dearest lord and brother, my lord Edward, by the grace of God king of England, his sister Mary wishes health, and all manner of honour and reverence." It ends "May Jesus Christ grant you a long life, my very dear brother."
Mary, the great lady forced to live as a nun, died at Amesbury on 29 May 1332 at the age of fifty-three, and was buried there. In 1344, John de Warenne, earl of Surrey, renewing his long-term efforts to divorce Jeanne de Bar, his wife of the last thirty-eight years, claimed to have had an affair with Mary before his marriage - his wife's aunt, thus making the marriage incestuous. The story is most unlikely to be true, not least because Mary was seven years older than Surrey. No doubt he chose her because she was so closely related to his wife, was dead, and had no children or surviving siblings to take offence at his claims. But it's pretty damn amusing anyway.