Margaret was the ninth or tenth child of Edward I and Eleanor of Castile, and the third to survive into adulthood. She was born at Windsor Castle on 15 March 1275, sixteen months younger than her brother Alfonso and thirteen and a half months older than her sister Berengaria, and nine years older than Edward II. As part of the huge confusion over the children of Edward and Eleanor, she was often identified in the past as 'Isabella'. Edward and Eleanor had no daughter of this name; their youngest daughter Elizabeth was occasionally called 'Isabella' as well. Margaret is often said to have been born on 11 September 1275, which is impossible, as her sister Berengaria was born in May 1276.
Margaret was betrothed in early childhood to the son and heir of Duke Jan I of Brabant and Margareta of Flanders. Duke Jan's son, also called Jan, inevitably, was born on 27 September 1275, so was six months younger than Margaret. Their betrothal was arranged in 1278, when they were both toddlers. Both King Edward and Duke Jan were very keen for the marriage to go ahead: Edward promised Jan I that if Margaret was not able to wed young Jan, her next eldest sister would be substituted instead, and Jan promised Edward that if his son did not marry Margaret, he would pay him 40,000 livres tournois, or £10,000 sterling. (The medieval duchy of Brabant covered part of modern-day Belgium and the Netherlands: see this map).
Young Jan of Brabant arrived in England in 1284, the year his future brother-in-law Edward II was born, when he was eight or nine, and lived there for the next ten years. In about 1285, he wrote in a letter to Edward I "Dear sire, I pray you to take council that I may marry soon, as I greatly desire it. Command me at your will as your son." (He was still only nine or ten). He sometimes lived in the same household as his future brother-in-law Edward of Caernarfon (maybe he played the role of elder brother?), sometimes with the king, sometimes with Edward I's Lancaster nephews, and sometimes alone. Jan was described by a contemporary as "stout, handsome, gracious, and well-made." (No description of Margaret, or indeed any of her sisters, exists.)
On 8 July 1290, Margaret and Jan married at Westminster Abbey. She was fifteen and a few months, he still only fourteen. The wedding was a splendid, lavish affair. Jan's retinue consisted of eighty knights and sixty ladies, wearing costumes of Brabant. Margaret's six-year-old brother Lord Edward of Caernarfon was followed by a retinue of eighty knights, her brother-in-law the earl of Gloucester's retinue was 103 knights and sixty ladies, and the other earls also brought huge retinues of their own. 700 knights and 1000 citizens of London took part in the procession, and the guests were entertained by 400 minstrels and musicians. The royal family changed clothes three times during the course of the day, and the highlight was a banquet held in Westminster Hall, where Lord Edward's personal cook presented an edible replica of a castle.
In subsequent letters to Duke Jan I, Edward I usually referred to the couple as "John, your son and ours, and his wife Margaret, our daughter and yours" (Johan vostre fiz e le nostre, & Margarete sa femme nostre fille e la vostre). In 1290, young Jan and Margaret sent a letter to her father, stating that they were using Jan's seal and the seal of her mother Queen Eleanor, as her own "seal is not well known" (en la nom la avauntdite Margarete (poor ceo que son seal n’est mie conu) avoms prie estre mis le seal la noble dame Alianore, reigne d’Engleterre). Jan was always called 'the king's son' after 1290, and called himself "his [Edward I's] son and son of the duke of Brabant."
In 1292/93, Jan was sharing the household of Thomas and Henry of Lancaster, nephews of Edward I and cousins of Margaret. They spent a few days staying with Jan's brother-in-law, nine-year-old Edward of Caernarfon, on several occasions. Although Jan and Margaret were now seventeen/eighteen, evidently they were still not living together.
In March 1294, Margaret and her brother Edward were very ill with the 'tertian fever', but fortunately both recovered. A few weeks later, on 3 May, Jan's father Duke Jan I was killed at a jousting tournament at Bar-le-Duc, arranged by Count Henri III of Bar to celebrate his marriage to Margaret's sister Eleanor. Eighteen-year-old Jan succeeded as duke of Brabant, and in late June 1294, returned to his homeland in a merchant's ship, sailing from Harwich.
However, Margaret remained in England for several years, and wasn't reunited her husband until early 1297, when Count Jan I of Holland married her sister Elizabeth, and she sailed with him. Margaret was presented with jewels before her departure, which she haughtily rejected, saying that they "did not please her." She had spent much of the time between June 1294 and January 1297 living with her brother Edward, and Elizabeth. The long separation seems to suggest that Margaret and Jan's marriage was not a particularly successful one. Jan consoled himself in her absence, and fathered four illegitimate sons, who, brilliantly, were all called Jan, and a daughter called Johanna. It's also possible that Jan's illegitimate children, or some of them, were born after he was reunited with Margaret, of course. Mary Anne Everett Green says in her Lives of the Princesses of England (1857) that Margaret was "doomed to the mortification of being perpetually surrounded by the bastard sons of her husband."
Margaret and Jan's only child, the future Jan III of Brabant, was born sometime in late 1300, when Margaret was twenty-five. The news was greeted with joy in England: Edward I gave a gift of a hundred marks (sixty-six pounds) to the messenger who informed him, Margaret's stepmother Queen Marguerite fifty marks, Edward of Caernarfon forty, and their sister Elizabeth twenty. (So, well worth the messenger's trouble in crossing to England.)
Duke Jan and Duchess Margaret attended the wedding of Edward II and Isabella of France at Boulogne on 25 January 1308, and also attended their coronation at Westminster a month later. This is, as far as I know, the last time Margaret ever visited England. She and Edward remained in contact, and Edward's relations with Brabant were warm throughout his reign, though somehow I get the impression that Edward was not as close to Margaret as he was to his other sisters. In October 1311, however, he asked Margaret and Jan to receive Piers Gaveston during his third exile.
Duke Jan II died on 27 October 1312, supposedly of kidney stones, at the age of only thirty-seven, and Margaret's twelve-year-old son succeeded as Jan III. A contemporary Flemish chronicle says, inexplicably, that Margaret died in 1318, and this is still often repeated today. However, it's perfectly clear that Margaret lived far beyond 1318, as she was still in touch with her brother via letters in the 1320s, and also her nephew Edward III.
In 1330, Margaret was still alive when the enemies of Queen Isabella and Roger Mortimer gathered in Brabant, plotting an invasion of England. The Fieschi Letter claims that Brabant was one of the places that the former Edward II visited after his escape from Berkeley Castle, in about 1331.
The date of Margaret's death is unknown, but she was still alive in March 1333, when she sent a letter to Edward III. At this time, she was almost fifty-eight. She was the last survivor of Edward I and Eleanor of Castile's children (except the non-murdered-in-1327 Edward II!). Of all Edward I's children, only her half-brother Thomas of Brotherton outlived her, with the same proviso regarding Edward II. Margaret was buried in Brussels, next to her husband.
Her only son Duke Jan III married Marie d'Évreux, niece of Philippe IV of France (her sister Jeanne married their first cousin Charles IV, Queen Isabella's brother, in 1325), and died on 5 December 1355. His three sons had all pre-deceased him, so he was succeeded by Johanna, the eldest of his three daughters. He also fathered a whopping twenty illegitimate children, which probably means that numerous people in the Low Countries today are descendants of Duchess Margaret.