I've looked before at the peculiar marriage of Margaret's father Thomas of Brotherton, earl of Norfolk and Earl Marshal of England, who was born on 1 June 1300 just under nine months after his father Edward I married his second wife Marguerite of France on 8 September 1299. Thomas was the son and half-brother of kings of England, the grandson and nephew of kings of France (Philip III and Philip IV), yet married the utterly obscure Alice Hales, whose father Sir Roger Hales was the coroner of Norfolk. The date of Thomas and Alice's wedding is not recorded, but it almost certainly took place after Thomas turned twenty-one and thus came of age on 1 June 1321. That it did take place after that date is pretty well confirmed by Edward II's efforts to find a more suitably royal bride for his half-brother. In 1320/21 he negotiated with King Jaume II of Aragon (r. 1291-1327) for Thomas to marry Jaume's youngest daughter, the widowed Violante (b. 1299), but in August 1321 Jaume told Edward that Violante had decided to become a nun.
Thomas is likely, therefore, to have made a love-match or a lust-match with the coroner of Norfolk's daughter sometime in or after August 1321. They had three children: in birth order, they were Margaret, Alice, and Edward, who died young. Given the probable date of her parents' wedding, Margaret is unlikely to have been born before the spring or summer of 1322, and as she gave birth to her daughter Elizabeth Segrave, Lady Mowbray, in October 1338, she is unlikely to have been born after 1323. Margaret's younger sister Alice was of age, i.e. fourteen as she was already married to the earl of Salisbury's brother Edward Montacute, by the time of their father's death in August 1338, and therefore must have been born in or before August 1324. Annoyingly, I've never found any references to the Norfolk sisters' births or baptisms in their uncle Edward II's extant accounts, though their mother Alice does appear occasionally as 'the king's sister, the Countess Marshal'.
I wonder if Margaret was just about old enough to be aware of the deposition and reported death of her uncle Edward II in 1327. On 3 March that year, Queen Isabella granted Thomas of Brotherton - who was both her brother-in-law and her first cousin - the marriage rights of John Segrave, son and heir of Stephen, Lord Segrave. Rather confusingly, Stephen Segrave died not long before 12 December 1325 mere weeks after his father John Segrave the elder died not long before 4 October 1325. Stephen's son John Segrave the younger was said to be either nine or ten years old in his father's inquisition post mortem held in late 1325 and early 1326. He was granted seisin of his inheritance on 20 March 1336 having recently proved his age, so was not born too long before 20 March 1315 and was a good seven years older than Margaret. 
Thomas of Brotherton married John Segrave to his elder daughter at an uncertain date, though the wedding definitely took place before Margaret was twelve or perhaps fourteen; we know this because she later complained to the pope that she was contracted to marry John before she was of marriageable age and never consented to cohabit with him (see below). Given that Margaret was the granddaughter and niece of kings, marrying Lord Segrave wasn't exactly the greatest marriage for her, and the marriage of her younger sister Alice to the earl of Salisbury's youngest brother wasn't that great either. This probably indicates that Thomas arranged the marriages of both his daughters before his young son Edward of Norfolk died, around 1333/34, at which point the girls became his heirs.
Margaret of Norfolk gave birth to her daughter Elizabeth Segrave, later Lady Mowbray, at Croxton Abbey in Leicestershire on 25 October 1338.  Her son John Segrave the younger was probably born around 1340 or 1341, and on or a little before 4 May 1347 was betrothed to Blanche of Lancaster (b. 1342), the younger daughter though ultimately the sole heir of Henry of Grosmont, earl of Lancaster, Leicester and Derby (and later the daughter-in-law of Edward III and mother of Henry IV). For some reason this did not work out, and on 25 March 1349 John Segrave was betrothed instead to Blanche Mowbray, future Lady Poynings, who was Henry of Grosmont's niece and Blanche of Lancaster's namesake first cousin; she was the daughter of Joan of Lancaster, one of Henry's six sisters. The young couple actually did marry, as in September 1353 there's a reference on the Close Roll to 'Blanche late the wife of John son of John de Segrave'.  Most confusingly, Margaret of Norfolk's nephew and niece Edward and Audrey Montacute, two of her younger sister Alice's five children, had previously been betrothed to Blanche Mowbray and her brother John, on 13 March 1342. 
Margaret's son John Segrave the younger died as a child sometime after his betrothal/marriage in March 1349; when exactly is not known, but when his father died in April 1353, his sister Elizabeth was named as their father's sole heir.  Before 4 September 1353, Elizabeth married John, Lord Mowbray (b. June 1340), brother of her brother's wife Blanche Mowbray and nephew of Henry of Grosmont, and formerly betrothed to Audrey Montacute. Elizabeth and John's second son Thomas Mowbray, probably born in March 1367 - his inquisition post mortem of 1399 gives March 1366, but his brother, who died as a teenager in 1383, was born in August 1365 - was made first duke of Norfolk in 1397 and ultimately was his grandmother Margaret of Norfolk's sole heir.
Douglas Richardson's book Plantagenet Ancestry gives Margaret of Norfolk another Segrave daughter called Margaret, and two sons, not one, called John Segrave. Margaret Segrave and the other John Segrave are said to have died young. This may well be the case, but Richardson's habit is to add a long list of sources to the end of each genealogical biography in his books with no indication as to which source belongs to which statement - the Oxford Dictionary of National Biography, annoyingly, does the same thing - so I have no idea which primary sources he used.
Elizabeth Segrave was born a couple of months after her royal maternal grandfather Thomas of Brotherton, earl of Norfolk, died in August 1338, and, as stated in her proof of age, her father Lord Segrave returned to England from the continent 'about the day of St. Matthew [21 September] before the said Elizabeth’s birth...because of his claim to the said earl's lands in right of Margaret his wife'. Thomas of Brotherton's daughters Margaret and Alice were named as his heirs in his IPM ('Margaret the wife of John de Segrave and Alice the wife of Edward de Monte Acuto are his next heirs'). 
By 10 June 1350, Margaret of Norfolk was making attempts to have her marriage to John Segrave annulled; on that date, the pope told Ralph Stratford, bishop of London, 'to hear and decide the cause of' John and Margaret, 'who was contracted to him before she was of marrriageable age and has never agreed to cohabit with him'. On 2 March 1351, the pope made references to the 'matrimonial cause between Margaret Marescallis of Lopham [in Norfolk] and John Segrave, knight'. 
Margaret's marriage was evidently an unhappy one, but at least it wasn't murderous like her unfortunate younger sister Alice's. Alice was terribly beaten by her husband Edward Montacute and at least two of his retainers, and died at the end of 1351 or beginning of 1352, probably after lapsing into a coma. Of Alice's five children, the youngest, Joan Montacute (b. 1349), was the only one who lived into adulthood and married; she wed William Ufford (b. 1338), earl of Suffolk. Joan and William had several children though none of them lived, and when William died in 1382, having held his mother-in-law Alice's share of the Norfolk inheritance by right of his late wife and the 'courtesy of England', his aunt-in-law Margaret of Norfolk was named as heir to the lands ('Margaret Mareschall, aged 50 years and more, is daughter and sole heir of Thomas, late earl of Norfolk and marshall of England, and aunt and next heir of Joan late the wife of the said earl of Suffolk').  She now held the entire Norfolk inheritance of her long-dead father Thomas of Brotherton.
Margaret's attempts to have her Segrave marriage annulled proved unsuccessful, even though a pardon granted to her by her cousin Edward III in late 1355 makes it apparent that she must have gone to the papal court in Avignon in person: she 'cross[ed] to foreign parts against his prohibition'. On 27 October 1350, Margaret 'crossed the Channel contrary to the king's prohibition...in a barge of William de Denum called le Faucoun...she was met at night by Thomas Barbour, servant of Sir Walter Manny'.  John Segrave died fairly soon afterwards anyway, on 1 April 1353, only in his late thirties. Soon afterwards, Margaret married her second husband, the Hainaulter knight Sir Walter Manny or Mauny (b. c. 1310), who had come to England in Queen Philippa's retinue. It seems almost certain that this was a love-match or at least a lust-match, and Margaret had certainly known and perhaps fallen in love with Walter during her unhappy marriage to John Segrave.
Margaret and Walter Manny's daughter Anne was, according to Walter's inquisition post mortem, born on 24 July 1354; she was 'aged seventeen on the eve of St James last' in February/March 1372.  Other jurors stated that she was eighteen in 1372. If they were correct, it would appear that Margaret of Norfolk became pregnant around late October or early November 1353, only about seven months after John Segrave's death. Anne Manny was the couple's only surviving child, though genealogist Douglas Richardson states that they had another daughter, Isabel, and a son Thomas as well, who drowned in a well at the age of ten, apparently. Or at the age of five, according to the Medieval Lands project on fmg.ac. Again, I have no idea what the source is.
On 26 July 1354, which would appear to have been just two days after Margaret gave birth to Anne Manny, Edward III ordered his cousin's arrest, and sent two sergeants-at-arms to 'lead her as quietly and honourably as they can' to Somerton Castle in Lincolnshire, which was given to Edward II by the bishop of Durham in 1309 and later passed to his son and heir. Margaret was imprisoned there for a time, though it was not an onerous incarceration; she was accompanied by her entire household, and Walter Manny was also allowed to stay there with her if he wished. Keenly aware of her status as his cousin and as a king's granddaughter, Edward ordered the castle constable to treat Margaret 'in all things in accordance with her estate'. The king pardoned her and Walter in late 1355 for 'all rancours and wraths conceived against them by the king for any causes', and for marrying without his licence. 
Sir Walter Manny hadn't, as far as I know, been previously married, though had fathered two illegitimate daughters whom he named in his will of 30 November 1371, and who had the best names ever: Mailosel and Malplesant. He also had a sister called Mary or Marie, and a cousin who bore the curious name of Cishbert (doesn't sound like a traditional Hainault name, does it?)  Walter died on 15 January 1372, and ten months later, on or about 11 November 1372, his and Margaret of Norfolk's eighteen-year-old daughter Anne gave birth to her only child, John Hastings the younger, named after her husband John Hastings, earl of Pembroke (b. August 1347).  Walter referred to his wife in his will as 'Margaret Mareschall' and left her, among other things, all his beds except a folding bed which he left to their daughter, a gold girdle, a gold garter, and a 'hook for a mantle'.
Margaret of Norfolk was probably not quite fifty years old when she was widowed for the second time, and never remarried. Her joint heirs were her two daughters, Elizabeth Segrave, Lady Mowbray (b. 1338), and Anne Manny, countess of Pembroke (b. 1354). Elizabeth died sometime in 1367 or 1368; the date is not recorded but she appears to have given birth to her son Thomas Mowbray in March 1367, and the IPM of her husband John Mowbray (who died overseas sometime between June and October 1368) makes it apparent that he had held all her lands after her death by the 'courtesy of England'.  Anne Manny was widowed in 1375, and died in June 1384, not quite thirty. Margaret was granted sole custody of her grandson John Hastings, heir to the earldom of Pembroke, who as a seven-year-old in June 1380 had been married to John of Gaunt's daughter Elizabeth of Lancaster, then sixteen or seventeen. 
Margaret's grandson John Mowbray, the elder son of Elizabeth Segrave and aged not quite twelve, was made the first earl of Nottingham at Richard II's coronation in July 1377, though died, still a teenager, in 1383, leaving his brother Thomas as his heir. Margaret became a great-grandmother in September 1385 when Thomas Mowbray, then eighteen years old, and his wife Elizabeth Fitzalan's son Thomas Mowbray the younger was born. Four years later in December 1389, her seventeen-year-old other grandson John Hastings was killed during the jousts held to celebrate Christmas, in the presence of Richard II and his queen, Anne of Bohemia. Thomas Mowbray, earl of Nottingham, was now her sole heir.
In September 1397, Richard II took his revenge on the three senior Lords Appellant who had executed or exiled many of his friends during the Merciless Parliament of 1388. The king rewarded his supporters with higher titles, and Thomas Mowbray, earl of Nottingham, though also one of the five Lords Appellant of nine years earlier, was made the first duke of Norfolk. His grandmother Margaret was made duchess of Norfolk in her own right, and though she shared the title with her granddaughter-in-law Elizabeth Fitzalan, she was the first English woman to be made a duchess in her own right, now aged seventy-five or thereabouts.
Margaret died on 24 March 1399, aged seventy-six or seventy-seven, apparently intestate, or at least I've never seen any will she left. She was the last surviving grandchild of Edward I; the second last was Margaret de Bohun, countess of Devon, who died at age eighty in December 1391. Margaret's sole heir was her thirty-two-year-old grandson Thomas Mowbray, who had been permanently exiled from England six months earlier by Richard II. Thomas outlived his grandmother by only six months and died of the plague in Venice on 22 September 1399, so Margaret's heir was her fourteen-year-old great-grandson Thomas Mowbray the younger (September 1385 - June 1405). Margaret was buried in the church of the Greyfriars in London, the same church as Edward II's queen Isabella of France (d. 1358), Isabella's daughter Joan of the Tower, queen of Scotland (d. 1362), and Isabella's granddaughter Isabella of Woodstock, countess of Bedford (d. 1382).
1) Calendar of Patent Rolls 1327-30, p. 23; Calendar of Inquisitions Post Mortem 1317-27, nos. 699, 700, 728; Calendar of Close Rolls 1333-37, p. 555.
2) CIPM 1352-60, no. 121.
3) The National Archives BCM/D/5/101/8; Calendar of Papal Letters 1342-62, p. 305; Petitions to the Pope 1342-1419, p. 151; CPR 1348-50, p. 373; CCR 1349-54, p. 557.
4) TNA BCM/D/1/1/9 and 11.
5) CIPM 1352-60, nos. 116, 121.
6) CIPM 1336-46, nos. 195-6.
7) CPL 1342-62, pp. 381, 391.
8) CIPM 1377-84, nos. 599-623.
9) CPR 1354-58, p. 325; Calendar of Inquisitions Miscellaneous 1348-77, no. 50.
10) CIPM 1370-73, no. 148.
11) CPR 1354-58, pp. 93, 325; CCR 1354-60, p. 27; TNA C 49/7/27.
12) Testamenta Vetusta, vol. 1, pp. 85-6.
13) John was 'aged two on the feast of St Martin last' in the summer of 1375 and 'aged eleven on the feast of St Martin in Winter last' in June 1384: CIPM 1374-77, no. 148; CIPM 1384-92, nos. 12, 13, 20.
14) CIPM 1365-69, no. 397.
15) CPR 1381-85, p. 437; TNA SC 8/125/6209-6211; SC 8/129/6440.