Æthelred, Lord of the Mercians became ruler of English Mercia shortly after the death or disappearance of its last king, Ceolwulf II in 879. Æthelred's rule was confined to the western half, as eastern Mercia was then part of the Viking-ruled Danelaw. His 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. This alliance was cemented by the marriage of Æthelred to Alfred's daughter Æthelflæd.

Mercia was one of the three main Anglic kingdoms founded after Sub-Roman Britain was settled by Anglo-Saxons in an era called the Heptarchy. It was centred on the River Trent and its tributaries, in a region now known as the Midlands of England.