Edmund I., the Magnificent. was born in 922, the twelfth of his father's fifteen children. The first of the six Boy Kings, he reigned from 939 to 946. He had to meet a general uprising of the Danes of Mercia as well as those of the North. In the suppression of this he showed himself to be a great statesman as well as a great warrior. Little is definitely known about the policy of the Scots at this time but it appears that they joined the English whenever they were afraid of the Danes, and joined the Danes whenever they were afraid of the English. Edmund made it to be the interest of the Scottish King permanently to join the English. The southern part of the kingdom of Strathclyde had for some time been under the English Kings. In 945 Edmund took the remainder, but gave it to Malcolm on condition that he should be his fellow worker by sea and land. The king of the Scots thus entered into a position of dependent alliance towards Edmund. A great step was thus taken; the dominant powers in the island were to be English and Scots, not English and Danes. Edmund thought it worth while to conciliate the Scottish Celts rather than to endeavor to conquer them. The result of Edmund's statesmanship was soon seen, but he did not live to gather its fruits.

In 939 following Edmund's brother (and predecessor) Athelstan's death, Olaf of Dublin re-invaded Northumbria and established himself again in York as king. In 940 he led his army south and seized the "Five Boroughs" of Danish Mercia. The Vikings of East Anglia, encouraged by Olaf's success, to Edmund's dismay, rebelled against English rule. Talks were held following the stalemate at the indecisive battle of Leicester, and a peace treaty was brokered whereby Edmund recognized Olaf as King of York [Danish Deira], and conceded to him the "Five Boroughs" of Danish Mercia and the Danish territories in East Anglia. Thus, at a stroke Edmund gave away the conquests of his predecessors, however, it was only a temporary set-back. Olaf [Godfred's son] was killed the next year, in 941, while attempting to conquer English Bernicia, and was succeeded in the Kingdom of York [Danish Deira] by [another] Olaf [Sihtric's son]. In 942, Edmund, determined to win back the territories he had lost, undertook a campaign against the Vikings. He showed himself to be a warrior-king like his predecessors. Edmund subdued the rebellious Vikings of East Anglia in 942, recovered the "Five Boroughs" of Mercia in 943, marched north into Northumbria, and by 944 brought Danish Deira back into subjection. Edmund, thus, recovered the territories that he had previously lost and restored the English state.

On May 26, 946 an outlaw named Lief, who had taken his seat at a banquet in his hall, slew him as Edmund was attempting to drag him out by his hair. He was succeeded by his brother Edred.