Waltheof was the second son of Siward, Earl of Northumbria. His mother was Aelfflaed, daughter of Ealdred, Earl of Bernicia, son of Uhtred, Earl of Northumbria. In 1054, Waltheof became the heir of his father's throne as his elder brother Osbearn was killed in battle. Siward died in 1055, and Waltheof being far too young to succeed as Earl of Northumbria, King Edward appointed Tostig Godwinson to the earldom.