Harald, born around 1040, as a son of king Sweyn II Estridsson, had taken part in Sweyn's 1069 raid of England alongside his uncle Jarl Asbjørn and his brother Canute, the later king Canute IV the Saint.
After the death of his father, Harald had been elected king in competition with his younger brother, Canute, at the Zealand Assembly at Isøre near Odsherred.
Accounts of the year include both 1074 and 1076, with the Monarchy of Denmark officially setting the year as 1074, two years before the death of Sweyn.
In order to get elected, he had taken the vows called Harald's laws, declaring his will to uphold the existing rule of law.
During his rule, Harald has met opposition from a number of his brothers, likely including Canute, who had enlisted the support of Olaf III of Norway.
Pope Gregory VII had mediated, advising Olaf not to take sides, and for Harald to share power with his brothers.
Canute, born around 1042 as one of the many illegitimate sons of Sweyn II Estridsson, is first noted as a member of Sweyn's raid of England in 1069.
The Anglo-Saxon Chronicle reports that Canute was one of the leaders of another raid against England in 1075.
When returning from England in 1075, the Danish fleet had stopped in the County of Flanders.
Because of its hostility towards William I of England, Flanders was a natural ally for the Danes.
He also led successful campaigns to Sember and Ester, according to skald Kálfr Mánason.
At the election of Harald III, Canute had gone into exile in Sweden, having possibly been involved in the active opposition to Harald, who, dependent on the great nobles of Denmark for his election, has done little to oppose them.
As a result, he has fought no major wars and spends his energy improving the few things that lie in his purview.
He is best known for improving and standardizing Danish coinage, and establishes mints at Ribe, Viborg, Lund, and Schleswig.
He institutes public use of the royal forests.
Harald also seeks to change Danish legal customs.
He deplores the ancient customs of trial by combat and the jernbyrd trial by ordeal of holding red-hot iron bars, and introduces a system used by the English of calling upon honorable men to swear oaths on behalf of the parties in a trial.
He allegedly continues Sweyn's politic of seeking a Danish Archbishopric with the Pope, but is not successful.
However, the legal reforms of Harald will not be fully accepted until the reign of Valdemar II the Victorious in the thirteenth century.
The historicity of his coinage reform has also been called into question.
Harald dies on April 17, 1080, and is interred at Dalby Church in Scania.
He is succeeded as king by his brother Canute IV, later called the Saint.
On his accession, he marries Adela, daughter of Count Robert I of Flanders.
