By his father's will he should have succeeded to Wessex on the death of his eldest brother Aethelbald (d. 860). He seems to have stood aside in favour of his brother Aethelbeht, king of Kent, to whose joint kingdoms he succeeded in 865 or 866.
Aethelred's reign was one long struggle against the Danes. In the year of his succession a large Danish force landed in East Anglia, and in the year 868 Aethelred and his brother Alfred went to help Burgred of Mercia against this host, but the Mercians soon made peace with their foes. In 871 the Danes encamped at Reading, where they defeated Aethelred and his brother, but later in the year the English won a great victory at a place called "Aescesdun." Two weeks later they were defeated at Basing but partially retrieved their fortune by a victory at "Maeretun" (perhaps Marden in Wiltshire), though the Danes held the field. In the Easter of this year Aethelred died, perhaps of wounds received in the wars against the Danes, and was buried at Wimborne. [Encyclopaedia Britannica]
King Alfred, under his will probably dated to [879/88], bequeathed estates at Aldingbourne, Compton, Crondall, Beeding, Beddingham, Burnham, Thunderfield and Earhing to "my brother's son Æthelhelm". He is named in the will before his brother Æthelwold, and received more extensive estates, suggesting that Æthelhelm was his father's older son. "Æthelhel[m] dux" subscribed the same undated charter of King Alfred as his brother Æthelwald, although curiously Æthelhelm is not given the epithet "filius regis" in the charter, in contrast to Æthelwald. Æthelhelm had [one possible child]: [ÆTHELFRITH (-904 or after).Anglo-Saxon Chronicle, E, 851
It may be possible to confirm family relationships with Æthelred I by comparing test results with other carriers of his Y-chromosome or his mother's mitochondrial DNA.
Y-chromosome DNA test-takers in his direct paternal line on WikiTree: