Æthelred (died 911) became Lord of the Mercians in England shortly after the death or disappearance of Mercia's last king, Ceolwulf II, in 879.

Mercia was the dominant kingdom in southern England in the eighth century, and maintained its position until it suffered a decisive defeat by King Ecgberht of Wessex at the Battle of Ellendun in 825.

Ecgberht briefly conquered Mercia, but it recovered its independence in 830, and thereafter the two kingdoms became allies, which was to be an important factor in English resistance to the Vikings.

The East Anglians were forced to buy peace, and the following year, the Vikings invaded Northumbria, where they established an obscure Northumbrian man called Ecgberht as puppet king in 867.

At almost the same time as Alfred's victory over the Vikings in 878 at the Battle of Edington, Ceolwulf defeated and killed Rhodri Mawr, king of the north Welsh territory of Gwynedd.

[10] In the view of Ian Walker: "He was a royal ealdorman whose power base lay in the south-west of Mercia in the former kingdom of the Hwicce around Gloucester.

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.

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.

[20] According to Frank Stenton, Alfred recovered London by force from the Vikings and handed it to Æthelred because it had previously been a Mercian town, and he respected the traditions of other kingdoms.

[21] Marios Costambeys takes a similar view, arguing that Alfred's decision was probably due to the need to maintain unity among the English who were outside Viking territory.

Æthelflæd's maternal uncle, Æthelwulf, controlled western and possibly central Mercia, while the south and east were ruled by Æthelfrith, the father of Æthelstan Half-King.

In 883, Æthelred freed Berkeley Abbey from obligations to the king's feorm (payments in kind), and in 887 he confirmed (with Æthelflæd also attesting) the possession of land and transferred manpower to Pyrton Minster in Oxfordshire.

In 901, they jointly gave land to Much Wenlock Abbey, and donated a gold chalice weighing thirty mancuses in honour of its former abbess, Saint Mildburgh.

Bailey cites "Mercian Register"[b] entries from 902 showing Æthelflæd acting alone or in conjunction with Edward in military operations.

[37] Irish annals called the Three Fragments also suggest that Æthelred was unable to take an active part in government from about 902,[38] although he did attend a meeting in 903 with King Edward, Æthelflæd and Ælfwynn.

[41] Martin Ryan also makes no mention of a decline in Æthelred's health, describing him as joining Edward in encouraging thegns to purchase land in Viking territories.

According to a transcript dating from 1304 in York's archives, in 925, Æthelstan gave a grant of privileges to St Oswald's Priory in Gloucester "according to a pact of paternal piety which formerly he pledged with Æthelred, ealdorman of the people of the Mercians".

Æthelflæd died in 918, and their daughter Ælfwynn briefly ruled Mercia until deposed by Edward the Elder, who took the territory under his direct control.

In 909, a West Saxon and Mercian army raided Viking territory and seized the bones of the Northumbrian king and martyr, St Oswald, from Bardney in Lincolnshire.

[46] The historian Martin Ryan sees the new minster as something like a Mercian royal mausoleum, to replace the one at Repton destroyed by the Vikings, and Æthelred and Æthelflæd were buried there.

[59] Keynes takes the West Saxon view, arguing that Alfred created the "kingdom of the Anglo-Saxons", inherited by his son Edward the Elder in 899, and Æthelred ruled Mercia under the king.