He supported Cnut's successors Harthacnut and Edward with vital military aid and counsel, and probably gained control of the middle shires of Northampton and Huntingdon by the 1050s.

No contemporary or near-contemporary biography has survived, and narratives from around the time of his life such as the Encomium Emmae and the Vita Ædwardi Regis scarcely mention him; historians depend on a few entries in the Anglo-Saxon Chronicle and comparable Irish sources.

[10] Siward's career in northern England spanned the reigns of four monarchs; from Cnut, Harold Harefoot, and Harthacnut, into the early years of Edward the Confessor.

[15] Northumbria had been united with the West Saxon English kingdom only in the 950s, by King Eadred, and subsequent control was exerted through the agency of at least two ealdormen, one to the north and one to the south of the River Tees.

[24] Legendary material incorporated in the Vita et passio Waldevi comitis (or simply Vita Waldevi), the hagiographic biography of Siward's son Waltheof, states that Siward was the son of a Scandinavian earl named Bjorn and provides a genealogy claiming that he was the descendant of a polar bear,[23] a commonplace piece of Germanic folklore.

[25] Historian Timothy Bolton has recently argued that the similarities between these genealogies is evidence of a shared family tradition between the descendants of Siward and Thorgil Sprakling.

There he encountered another dragon, before meeting an Oðinn-like old man on a hill,[28] who handed him a raven banner and instructed him to proceed to London to receive the patronage of the king of England.

[31] Charters dating to 1019, 1024, 1032, 1033 and 1035 mention a Si[ge]ward Minister, "the thegn Siward", but it is impossible to securely identify any of these names with the man who became Earl of Northumbria.

[32] The earliest certain contemporary record of Siward occurs in a charter of King Cnut to Ælfric Puttoc, Archbishop of York, in 1033.

[34] Although William of Malmesbury asserted that Erik was driven back to Scandinavia, Scandinavian tradition firmly maintained he died in England.

[35] Historian William Kapelle believed that Erik ceased to be earl in or soon after 1023, and that Carl son of Thurbrand was appointed hold or high-reeve (heahgerefa) for the king in Yorkshire.

[37] Timothy Bolton, although rejecting Kapelle's argument concerning Carl son of Thurbrand, believed Erik died c. 1023 and that the earldom may have remained vacant for a period.

[38] Bolton argued that Cnut left the earldom of Northumbria empty and appears to have paid it little attention until the last years of his reign, and another northerner Ealdred son of Uhtred rose to power in the political vacuum.

[49] Kapelle has pointed out that no ruler of Bamburgh after Uhtred is attested at the English royal court, which he argued "must mean they were in revolt" against the monarchy, and that Siward's attack may therefore have been encouraged by a monarch wishing to crush a rebellious or disloyal vassal.

As an ætheling, a royal prince with a present or likely future claim on the throne, Edward appears to have been invited back by Harthacnut in 1041, fortuitously smoothing over the coming change in ruler.

[61] The Anglo-Saxon Chronicle relates that although Siward had to call up reinforcements, King Edward was successful and Earl Godwine was temporarily exiled.

[62] Earl Godwine remained a threat in exile, and the continued "belligerent support" of Siward and Leofric was thus vital to King Edward's safety.

[77] According to the Libellus, the clergy were "terrified and overwhelmed by the fearful power of the earl" and "were compelled willy nilly to be reconciled to the bishop, and to admit him into his episcopal see".

[83] Possibly Siward's last historical appearance in English legal documents is in the agreement made—probably at Lincoln—between Wulfwig, Bishop of Dorchester, and Earl Leofric, dating to between 1053 and 1055.

According to the Libellus de Exordio, in 1039 or 1040, the Scottish king Donnchad mac Crínáin attacked northern Northumbria and besieged Durham.

[87] The failed siege occurred a year before Siward attacked and killed Earl Eadwulf of Bamburgh, and though no connection between the two events is clear it is likely that they were linked.

[88] The Annals of Lindisfarne and Durham, written in the early 12th century, relate under the year 1046 that "Earl Siward with a great army came to Scotland, and expelled king Macbeth, and appointed another; but after his departure Mac Bethad recovered his kingdom".

[89] Historian William Kapelle thought that this was a genuine event of the 1040s, related to the Annals of Tigernach entry for 1045 that reported a "battle between the Scots" which led to the death of Crínán of Dunkeld, Donnchad's father; Kapelle thought that Siward had tried to place Crínán's son and Donnchad's brother Maldred on the Scottish throne.

[102] Duncan's argument has been supported by several subsequent historians specialising in the era, such as Richard Oram, Dauvit Broun and Alex Woolf.

[106] It has been suggested that the chief consequence of Siward's expedition was not the overthrow of Mac Bethad, but the transfer of British territory—perhaps previously lying under Scottish suzerainty—to Northumbrian overlordship.

Alex Woolf has posited that, in such a context, Máel Coluim might have been a discontented Cumbrian prince who had been forced to "put himself under English protection".

[110] The Vita Ædwardi Regis states that Siward died at York and was buried in "the monastery of St Olaf" at Galmanho, a claim confirmed by the Anglo-Saxon Chronicle, John of Worcester, and the Historia Regum.

[111] Material incorporated in two surviving sources is thought by some to attest to the existence of a lost saga or some other kind of literary tradition concerning Siward's life.

The second major witness of the tradition is Henry of Huntingdon's Historia Anglorum, which contains extracts of saga-like material relating to Siward's invasion of Scotland (1054) and his death (1055).

[118] Moreover, the counties that would become Durham, Northumberland, Cumberland and Westmorland were largely omitted from the survey, while, besides being only very poorly documented, the lands in Yorkshire had been severely devastated and devalued during the Harrying of the North.