Waltheof is the second son of Siward, Earl of Northumbria.

His mother was Aelfflaed, daughter of Ealdred, Earl of Bernicia, son of Uhtred, Earl of Northumbria.

Waltheof’s brother, Osbearn, who was much older than he, had been killed in battle in 1054, making Waltheof his father’s heir.

Siward himself had died in 1055, and Waltheof being far too young to succeed as Earl of Northumbria, King Edward had appointed Tostig Godwinson to the earldom.

He is said to be devout and charitable and was probably educated for a monastic life.

Instead, around 1065 he had become an earl, governing Northamptonshire and Huntingdonshire.

Following the Battle of Hastings he had submitted to William and had been allowed to keep his pre-Conquest title and possessions, remaining until 1068 at William’s court.

When Sweyn II invaded northern England in 1069, Waltheof and Edgar Ætheling had joined the Danes and had taken part in the attack on York.

He had again made a fresh submission to William after the departure of the invaders in 1070.

Restored to his earldom, and went on to marry William's niece, Judith of Lens, daughter of Lambert II, Count of Lens and Adelaide of Normandy, Countess of Aumale.

He had in 1072 been appointed Earl of Northampton.

William in 1072, had expelled Gospatric from the earldom of Northumbria.

Gospatric is Waltheof’s cousin and had taken part in the attack on York with him, but like Waltheof, had been pardoned by William.

Gospatrichad fled into exile and William had appointed Waltheof as the new earl.

Waltheof has many enemies in the north, among them members of a family who had killed Waltheof’s maternal great-grandfather, Uchtred the Bold, and his grandfather Ealdred.

This is part of a long-running blood feud.

Waltheof in 1074 moves against the family by sending his retainers to ambush them, succeeding in killing the two eldest of four brothers.