He acted as regent 1103-1104 for his father while he was on pilgrimage to Jerusalem alongside Archbishop Asser of Lund. As regent, he was courageous, but violent, cruel and debauched. Harald plundered far and wide from his stronghold Haraldsborg at Roskilde, which greatly contributed to him not being elected king after his father died in 1103. Instead, his uncle Niels of Denmark was elected king in 1104.
In 1132, he allied with his half-brother Eric Emune in order to avenge his third brother Canute Lavard, but he later turned to the murderer Magnus I of Sweden because he had desires for the Danish throne himself. He tried to gain recognition as king by the name of Harald IV and was successful in receiving the homage of Jutland, but he was defeated along with Magnus Nilsson at the Battle of Fotevik in Skåne, 1134. He fled after the defeat but he was taken captive near Vejle on Jutland and decapitated together with eight of his sons.