Coenwulf was the King of Mercia from December 796 until his death in 821. He was a descendant of King Pybba, who ruled Mercia in the early 7th century. He succeeded Ecgfrith, the son of Offa; Ecgfrith only reigned for five months, and Coenwulf ascended the throne in the same year that Offa died. In the early years of Coenwulf's reign he had to deal with a revolt in Kent, which had been under Offa's control. Eadberht Præn returned from exile in Francia to claim the Kentish throne, and Coenwulf was forced to wait for papal support before he could intervene. When Pope Leo III agreed to anathematise Eadberht, Coenwulf invaded and retook the kingdom; Eadberht was taken prisoner, was blinded, and had his hands cut off. Coenwulf also appears to have lost control of the kingdom of East Anglia during the early part of his reign, as an independent coinage appears under King Eadwald. Coenwulf's coinage reappears in 805, indicating that the kingdom was again under Mercian control. Several campaigns of Coenwulf's against the Welsh are recorded, but only one conflict with Northumbria, in 801, though it is likely that Coenwulf continued to support the opponents of the Northumbrian king Eardwulf.

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.