She was born the daughter of Thomas Brotherton, Earl of Norfolk, and Alice Hales. She had two younger siblings, Edward and Alice. It was Edward’s death sometime before late 1334 that made Margaret her father’s heir. In 1335, when she was about 15, she married Baron John Segrave. Three years later, her father died, leaving her the earldom of Norfolk.
Whether or not Margaret ever actually acted as Earl Marshal is unknown. At this point in time, most of the responsibilities of the office pertained mostly to overseeing the care of the king’s horses, though it was in this century that the position became associated with law courts as well. We have no record of her discharging any of these responsibilities. Her request to do so in 1377 was ignored and Henry Percy was appointed to the office.
In 1350, Margaret demanded a divorce from her husband on the grounds that she had been betrothed before she was of an age to consent* and had never agreed to live with him. To obtain this sort of separation, however, Margaret had to get the consent of the Pope, which would go easier if she traveled to Rome. King Edward III forbade her to go, but with a letter of safe conduct from the King of France and the help of Sir Walter de Mauny, she crossed the Channel in secret.
She never made it to Rome and her case was heard at Poitiers. In 1353, however, John Segrave died before anything could actually be finalized,. A year later, Margaret once again ignored Edward’s wishes and married Walter de Mauny without the King’s license. It took her some time to get back into the King’s good graces after that, but there was little he could do. In 1397 his successor Richard II created her Duchess of Norfolk** for life. She died two years later in 1399, with only daughters as her heirs.
*According to Church law, at least, this was a valid reason to grant a divorce as it was a violation of the doctrine of consent.
**As opposed to merely Countess which she had been before.
Richardson, Douglas. Plantagenet Ancestry: A Study in Colonial and Medieval Families. Salt Lake City, 2011.
Margaret, Duchess of Norfolk - Wikipedia