William, Count of Mortain

William of Mortain (bef. 1084–aft. 1140) was Count of Mortain and the second Earl of Cornwall of 2nd creation.

Life William was the son of Robert, Count of Mortain, the half-brother of William I of England and Maud de Montgomery, daughter of Roger de Montgomerie, 1st Earl of Shrewsbury and Mabel de Bellême. William was born before 1084.

From childhood, he harboured a bitter dislike for his cousin Henry I of England, and proudly demanded from him not only his father's earldoms of Mortain and Cornwall, but his uncle Odo, Bishop of Bayeux's Earldom of Kent. Clues to the character of William are to be found in the Hyde Chronicle, calling him "incorrigibly turbulent" and in William of Malmesbury's depiction of William as having "shameless arrogance." Henry in turn gave her in marriage to Eustace III, Count of Boulogne. Henry continued to stall William's demands until he had dealt with the Montgomerys, William's uncles, and exiled them from England.

He was captured with Duke Robert at the Battle of Tinchebrai (1106) and stripped of Mortain. William was imprisoned for many years in the Tower of London, and in 1140 became a Cluniac monk at Bermondsey Abbey, just across the River Thames from the Tower of London.