John of Peterborough says that Waltheof was given the Earldoms of Huntingdon and Northampton at his father's death; but as these were held by Tosti, the probability seems to be that he succeeded on the deposition of that Earl. Simeon of Durham says that he governed Bernicia as his father's deputy, but this seems improbable on account of his age, and is not confirmed by other authorities. On the accession of Harold, Tosti, in conjunction with Harold Hardrada, invaded Northumbria, but were defeated by Harold at Stamford Bridge. It was, however, the cause of the ruin of Harold, who, whilst banquetting at York in celebration of his victory, had news brought him that Duke William of Normandy had landed in Sussex, and he had to lead his army by forced marches to the south, arriving in the front of the fresh Norman troops footsore and wearied, and with the loss of many who had fallen out of the ranks during the march; the result being his defeat and death, which might have been otherwise but for this fatal expedition to York. The brother Earls, Morkere of Northumbria and Eadwine of Mercia, and Waltheof undertook to bring bodies of soldiers to his aid, but the former two stood aloof, from politic motives; but Waltheof sent his contingent, if he were not present at the battle himself, which is uncertain. Duke William was now King of England. London, with the south and east, had submitted at once, but it cost him some efforts to subjugate the west, and still more the north. He did, however, eventually make himself master of Yorkshire and the northern counties, built a castle at York, and placed therein William Malet as military governor of the city. The year after his accession, he found it necessary to visit his Norman Dukedom, when, fearing to leave behind him men so powerful, and whom he suspected of disaffection, he courteously invited Earls Eadwine, Morkere, and Waltheof, to accompany him as guests, who complied with his request, although they were perfectly aware that they were going as hostages for the good behaviour of their people during his absence. Soon after their return, the three Earls, under Earl Gospatric, made a demonstration in the north in favour of Eadgar, the Atheling, but were defeated, and fled to the court of Malcolm, in Scotland. William sent a herald to demand the fugitives, but the King declined giving them up.