Ealdorman Æthelred (died 911) was ruler of Mercia from about 883, when he submitted to King Alfred of Wessex. He married Alfred's daughter Æthelflaed between 882 and 887, and his title was "Lord of the Mercians". Although he had many attributes of a king, he was subject to the power of his close ally Wessex. His descent is unknown, and he does not appear to have been closely related to his immediate predecessors.

In 886, he was given control of London by Alfred. As Alfred reconquered the Danelaw, it was useful to place the ruler of the divided Kingdom of Mercia in control of its former eastern region.

The Anglo-Saxon Chronicle says that Æthelred spent much of the 890s leading military campaigns in eastern England. He was recorded as fighting many battles against the Welsh, and at one point Brochwael ab Meurig, joint ruler with his brother of the Welsh Kingdom of Gwent appealed to Alfred for help against Ealdorman Æthelred. From about 902, Æthelflaed became the effective ruler of Mercia due to Æthelred's ill health, and when he died in 911 she succeeded him. After Æthelflaed's death in 918, their daughter Ælfwynn briefly ruled Mercia until deposed by Edward the Elder.

Mercia ( /ˈmɜrsiə/, /ˈmɜrʃə/) was one of the kingdoms of the Anglo-Saxon Heptarchy. It was centred on the valley of the River Trent and its tributaries in the region now known as the English Midlands. The name is a Latinisation of the Old English Mierce or Myrce, meaning "border people".

Mercia's neighbours included Northumbria, Powys and the kingdoms of southern Wales, Wessex, Sussex, Essex and East Anglia.

The name Mercia is still in use today by a wide range of organisations, including military units, public, commercial and voluntary bodies; and occurs as a female given name.

Mercia's exact evolution from the Anglo-Saxon invasions is more obscure than that of Northumbria, Kent, or even Wessex. Also, Mercia developed an effective political structure and adopted Christianity later than the other kingdoms. Archaeological surveys show that Angles settled the lands north of the River Thames by the 6th century. The name Mercia is Old English for "boundary folk" (see Welsh Marches), and the traditional interpretation is that the kingdom originated along the frontier between the native Welsh and the Anglo-Saxon invaders. However, P. Hunter Blair argued an alternative interpretation: that they emerged along the frontier of Northumbria and the inhabitants of the Trent river valley.