On his father's death, in 1035, he succeeded as King of Denmark, but a dispute with Magnus I of Norway meant he did not sail to England to claim that throne, but left his elder half-brother Harald Harefoot to rule there as his regent. Harald elected, in 1037, to take the throne in his own right, but, in 1040, whilst Hardicanute was on his way with an army to invade and depose him, Harald died. Hardicanute invaded anyway, had his brother disinterred, and had him hurled into a fen.
To pay for the invasion, he taxed the English heavily (they had, after all, accepted Harald as king, and necessitated all the expense). This made him unpopular and shrewdly he enlisted another half-brother, Edward, to come over from Normandy to become his co-ruler. This was in around 1041, and just in time for Hardicanute, in July 1042, to suffer a seizure, rendering him dumb and then dead. He was buried at Winchester, and Edward, of good Saxon royal stock, assumed the throne of England, whilst Magnus of Normway took over in Denmark.