He was the son of Ulf Thorgilsson and Estrid Svendsdatter, and the grandson of Sweyn Forkbeard through his mother's line.

He was married at least two times, and fathered 20 children or more out of wedlock, including the five future kings Harald Hen, Saint Canute, Oluf Hunger, Eric Evergood, and Niels.

Sweyn grew up a military leader, and served under king Anund Jacob of Sweden for a time.

[5] When Harthacnut died in 1042, Magnus claimed the Danish throne and made Svend the jarl of Jutland.

[6] In 1043, Sweyn fought for Magnus at the Battle of Lyrskov Heath at Hedeby, near the present-day border of Denmark and Germany.

[5] Sweyn won a great reputation at Lyrskov Heath, and had the Danish nobles crown him king in Viborg in Jutland.

[4] In 1047 Magnus died, having stated on his deathbed that his kingdom would be divided: Harald would get the throne of Norway, while Sweyn would be king of Denmark.

[4] According to the sagas Harald urged Sweyn to meet him in a final and decisive battle at Elv in the spring of 1062.

[11] Sweyn managed to escape the battle, reached land and stopped at the house of a peasant to ask for something to eat.

He was an ally of Emperor Henry III against Count Baldwin V of Flanders in 1049, and Sweyn assisted his son-in-law Gottschalk in the Liutizi Civil War of 1057.

However, after capturing York, Sweyn accepted a payment from William to desert Edgar, who then returned into exile in Scotland.

He brought scholars to Denmark to teach him and his people Latin so they could converse with the rest of Europe on equal terms.

[citation needed] King Sweyn died at the royal estate Søderup, 10 kilometres (6.2 mi) west of Åbenrå at the Little Belt strait.

[3][2] The king's body was carried to Roskilde Cathedral where he was interred in a pillar of the choir next to the remains of Bishop Vilhelm (who was the actual person who died in 1074).

[14] One of the legacies of King Sweyn was a fundamental change in Danish society which had been based on whether a person was free or a bondsman.

[5] Sweyn sought to create a Nordic Archbishopric under Danish rule, a feat which his son Eric I accomplished.

[12] Sweyn seems to have been able to read and write, and was described as an especially educated monarch by his personal friend Pope Gregory VII.

[12] He is the source of much of our current knowledge about Denmark and Sweden in the 9th and 10th centuries, having told the story of his ancestry to historian Adam of Bremen around 1070.