The son of King Ethelred II the Unready (reigned 978–1016), Edmund defied his father's orders by marrying (1015) the widow of one of the Danish lords then occupying English territory, probably in order to enhance his chances at succession. Nevertheless, when Canute (Canute (I)) invaded England later in 1015, Edmund raised an army in northern England and ravaged regions that would not rally to his cause.

Upon Ethelred's death (April 1016), a small number of councillors and citizens of London proclaimed Edmund as their ruler, but a larger body of nobles at Southampton declared for Canute. Edmund then launched a series of offensives against his rival. He recovered Wessex and relieved London of a siege before being decisively defeated by Canute at Ashington, Essex, on October 18. In the ensuing peace settlement, Edmund retained Wessex, while Canute held the lands north of the River Thames. After Edmund died (probably of natural causes), Canute became sole ruler of England.