William the Conqueror ruled England for 21 years after the Norman victory at Hastings. He was crowned William I on Christmas Day 1066 and remained king until his death on 9 September 1087.
The Norman victory did not instantly secure the whole kingdom. William faced rebellions in several regions, including the north, and used castles, land grants and military campaigns to consolidate his rule. By about 1075, his control of England was sufficiently strong for him to spend much of his time in Normandy.
The figure is conventionally given as 21 years because William’s English reign ran from 1066 to 1087. Strictly, the interval from his coronation to his death was just under 21 calendar years, while the Battle of Hastings occurred several weeks before his coronation. William was succeeded in England by his son William II, known as William Rufus.