William de Mortaigne, Count of Mortain, Earl of Cornwall (d. aft. 1140) was the son of Robert, Count of Mortain

Robert, Count of Mortain

Robert, Count of Mortain, 1st Earl of Cornwall was a Norman nobleman and the half-brother of William I of England. Robert was the son of Herluin de Conteville and Herleva of Falaise and was full brother to Odo of Bayeux. The exact year of Robert's birth is unknown Robert, Count of Mortain, 1st...

, the half-brother of William I of England

William I of England

William I , also known as William the Conqueror , was the first Norman King of England from Christmas 1066 until his death. He was also Duke of Normandy from 3 July 1035 until his death, under the name William II...

. He was sometimes called in English William, Earl of Moriton, for instance by William Camden

William Camden

William Camden was an English antiquarian, historian, topographer, and officer of arms. He wrote the first chorographical survey of the islands of Great Britain and Ireland and the first detailed historical account of the reign of Elizabeth I of England.- Early years :Camden was born in London...

.

From childhood, he harbored a dislike for his cousin Henry I of England

Henry I of England

Henry I was the fourth son of William I of England. He succeeded his elder brother William II as King of England in 1100 and defeated his eldest brother, Robert Curthose, to become Duke of Normandy in 1106...

, and proudly demanded from him not only his father's earldoms of Mortain and Cornwall

Earl of Cornwall

The title of Earl of Cornwall was created several times in the Peerage of England before 1337, when it was superseded by the title Duke of Cornwall, which became attached to heirs-apparent to the throne.-Earl of Cornwall:...

, but his uncle's Earldom of Kent

Earl of Kent

The peerage title Earl of Kent has been created eight times in the Peerage of England and once in the Peerage of the United Kingdom.See also Kingdom of Kent, Duke of Kent.-Earls of Kent, first creation :*Godwin, Earl of Wessex...

. In 1103, he crossed from England

England

England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...

 into Normandy

Normandy

Normandy is a geographical region corresponding to the former Duchy of Normandy. It is in France.The continental territory covers 30,627 km² and forms the preponderant part of Normandy and roughly 5% of the territory of France. It is divided for administrative purposes into two régions:...

 and openly revolted against Henry in 1104, losing his English fiefs in consequence. He was captured with Robert Curthose at the Battle of Tinchebrai (1106) and stripped of Mortain. William was imprisoned for many years, but later escaped, and became a Cluniac monk

Monk

A monk is a person who practices religious asceticism, living either alone or with any number of monks, while always maintaining some degree of physical separation from those not sharing the same purpose...

 at Bermondsey Abbey

Bermondsey Abbey

Bermondsey Abbey was an English Benedictine monastery. Most widely known as an 11th-century foundation, it had a precursor mentioned in the early 8th century, and was centred on what is now Bermondsey Square, the site of Bermondsey Market, Bermondsey in the London Borough of Southwark, southeast...

in 1140.