Æthelstan probably did not approve of the increasing influence of ealdorman Eadric Streona from 1007, and he seems to have formed a friendship with Sigeferth and Morcar, two of the leading thegns of the Five Boroughs of the East Midlands.[3] Æthelstan and Edmund were close, and they probably felt threatened by Emma's ambitions for her sons.[4] The Life of Edward the Confessor, written fifty years later, claimed that when Emma was pregnant with him, all Englishmen promised that if the child was a boy they would accept him as king.[1] When Sweyn Forkbeard seized the throne at the end of 1013 and Æthelred fled to France, the brothers do not appear to have followed him, but stayed in England. Æthelstan died in June 1014 and left his brother estates and a sword which had belonged to king Offa of Mercia.[1] His will also reflected the close relationship between the brothers and the nobility of the east midlands.[5]

Æthelred died on 23 April 1016, and the citizens and councillors in London chose Edmund as king and probably crowned him. He then mounted a last-ditch effort to revive the defence of England. While the Danes laid siege to London, Edmund headed for Wessex, where the people submitted to him and he gathered an army. He fought inconclusive battles against the Danes and their English supporters at Penselwood in Somerset and Sherston in Wiltshire. He then raised the siege of London and defeated the Danes near Brentford. They renewed the siege while Edmund went to Wessex to raise further troops, returning to again relieve London, defeat the Danes at Otford, and pursue Cnut into Kent. Eadric Streona now went over to Edmund, but at the decisive Battle of Assandun on 18 October, Eadric and his men fled and Cnut decisively defeated Edmund. There may have been one further battle in the Forest of Dean, after which the two kings negotiated a peace dividing the country between them. Edmund received Wessex while Cnut took Mercia and probably Northumbria.[1]

Shortly afterwards, on 30 November 1016, King Edmund died, probably in London. Cnut was now able to seize control as king of England. Edmund was buried at Glastonbury Abbey in Somerset. His burial site is now lost. During the Dissolution of the Monasteries, any remains of a monument or crypt were destroyed. The location of his body is unknown.