Æthelred, Lord of the Mercians (or Ealdorman Æthelred of Mercia) (died 911) became ruler of English Mercia shortly after the death of its last king, Ceolwulf II in 879. His rule was confined to the western half, as eastern Mercia was then part of the Viking-ruled Danelaw. Æthelred's ancestry is unknown. He was probably the leader of an unsuccessful Mercian invasion of Wales in 881, and soon afterwards he acknowledged the lordship of King Alfred the Great of Wessex. The alliance was cemented by the marriage of Æthelred to Alfred's daughter Æthelflæd.

In 886 Alfred took possession of London, which had suffered greatly from several Viking occupations; as it had traditionally been a Mercian town, he handed control to Æthelred. In 892 the Vikings renewed their attacks, and the following year Æthelred led an army of Mercians, West Saxons and Welsh to victory over a Viking army at the Battle of Buttington. He spent the next three years fighting them alongside Alfred's son, the future King Edward the Elder. At some time in the decade 899 to 909, Æthelred's health may have declined, and Æthelflæd may have become the effective ruler of Mercia.

After Æthelred's death, Æthelflæd ruled as Lady of the Mercians until her own death in 918. The couple's only child, a daughter called Ælfwynn, then ruled briefly until deposed by her uncle, King Edward.

Æthelred's descent is unknown, and he does not appear to have been closely related to his immediate predecessors, although his name suggests possible descent from earlier Mercian kings.[7] He may have been related to King Alfred's Mercian father-in-law, Æthelred Mucel, and brother-in-law, Æthelwulf, who appears to have been a member of Æthelred's court from the mid 880s.[8][9] Æthelred may have been the man of the same name who attested two Mercian charters in the late 860s,[10] but he is not listed in the two surviving charters of Ceolwulf. Lists of witnesses to charters show that Æthelred's witan (council) shared bishops and at least two ealdormen with Ceolwulf, but Celowulf's thegns all disappeared.[8]

It is not known when Æthelred took over following Ceolred's death or disappearance, but in the view of Thomas Charles-Edwards, a historian of medieval Wales, Æthelred was almost certainly "Edryd Long-Hair", the leader of a Mercian army which invaded Gwynedd in 881, and was defeated by Rhodri Mawr's sons at the Battle of the Conwy. This was described by Welsh annals as "revenge by God for Rhodri". The defeat forced Æthelred to abandon his ambitions in north Wales, but he continued to exercise overlordship over the south-eastern Welsh kingdoms of Glywysing and Gwent.[11] According to Alfred's Welsh biographer Asser, Æthelred's "might and tyrannical behaviour" forced these kingdoms to submit to the protection of King Alfred's lordship.[12] By 883, Æthelred had accepted Alfred's lordship. Charles-Edwards suggests that in 881–882 he tried to maintain his dominance in south-east Wales, but Alfred offered his protection to Glywysing and Gwent, and in 882–883 Æthelred accepted that West Saxon power made continued independence impossible. Charles-Edwards comments:

The implication of all this is that the Mercian submission to Alfred – a crucial step in the creation of a single English kingdom – occurred not just because of one battle, Alfred's victory over the Great Army at Edington in 878, but also because of another, more distant battle, "God's revenge" on the Mercians at the Conwy, when Anarawd of Gwynedd and his brothers defeated Æthelred and so brought about that collapse of the Mercian hegemony in Wales from which Alfred was only too pleased to benefit.[13]
When Æthelred made a grant to Berkeley Abbey in 883, he did it with the approval of King Alfred, thus acknowledging Alfred's lordship.[5] Thereafter he usually acted with Alfred's permission, but issued some charters in his own name without reference to Alfred, such as at a meeting in Risborough in Buckinghamshire in 884, showing