She was the daughter of count Baldwin V of Flanders, and Adela Capet (1000-1078/9), daughter of Robert II of France.
A spoiled young lady used to speaking her mind and getting her way, the 4'2"-tall Matilda (or "Maud") told the representative of William, Duke of Normandy (later king of England as William the Conqueror), who had come asking for her hand, that she was far too high-born (being descended from King Alfred the Great of England) to consider marrying a bastard. When that was repeated to him, William, all 5'10" of him, rode from Normandy to Bruges, found Matilda on her way to church, dragged her off her horse (some said by her long braids), threw her down in the street in front of her flabbergasted attendants, and then rode off. After that, she decided to marry him, and even a papal ban (on the grounds of consanguinity) did not dissuade her.
There were rumors that Matilda had been in love with the English ambassador to Flanders, a Saxon so pale he was nearly an albino, named Brihtric (but nicknamed "Snow"), who was already married. Whatever the truth of the matter, years later when she was acting as regent for William in England, she used her authority to confiscate Brihtric's lands (without even any formal charges, much less a trial) and throw him into prison, where he died under suspicious circumstances consistent with poisoning.
When William was preparing to invade England, Matilda outfitted a ship, the Mora, out of her own money and gave it to him. For many years it was thought that she had something to do with creating the Bayeux Tapestry, but historians no longer believe that; it seems to have been commissioned by William's half-brother Odo, Bishop of Bayeux, and made by Saxons in Kent.
Matilda bore William ten children, and he was believed to have been faithful to her, at least up until the time their son Robert rebelled against his father and Matilda sided with Robert against William. After she died, in 1083 at the age of 51, William became tyrannical, and people blamed it on his having lost her. She was buried at St. Stephen's in Caen, Normandy (then, France now), and William was eventually buried there, too. Years later their graves were opened and their bones measured, which is how we know how tall they were.