Æthelred I was King of Wessex from 865 to 871. He was the fourth son of King Æthelwulf of Wessex. He succeeded his brother, Æthelberht (Ethelbert), as King of Wessex and Kent in 865. He may have acted as an underking as early as 862, and in 862 and 863 he issued charters as King of the West Saxons. This must have been as deputy or in the absence of his elder brother, King Æthelberht, as there is no record of conflict between them and he continued to witness his brother's charters as a king's son in 864.

In the same year as Æthelred's succession as king (865), a great Viking army arrived in England, and within five years they had destroyed two of the principal English kingdoms, Northumbria and East Anglia. In 868 Æthelred's brother-in-law, King Burgred of Mercia, appealed to him for help against the Vikings. Æthelred and his brother, the future Alfred the Great, led a West Saxon army to Nottingham, but there was no decisive battle, and Burgred bought off the Vikings. In 874 the Vikings defeated Burgred and drove him into exile.

In about 867, Æthelred effectively established a common currency between Wessex and Mercia by adopting the Mercian type of lunette penny, and coins minted exclusively at London and Canterbury then circulated in the two kingdoms.

Æthelred died shortly after Easter (15 April) 871, and is buried at Wimborne Minster in Dorset. He was succeeded by his younger brother, Alfred the Great due to Æthelred's sons being too young.   (Wikipedia).