William of Malmesbury declared that, after Cœnwulf: "the kingdom of the Mercians declining, and if I may use the expression, nearly lifeless, produced nothing worthy of historical commemoration." Actually, Mercia did have a moment of glory that William was unaware of. Indicating the year 822, the Annales Cambriae states: "The fortress of Degannwy (in Gwynedd) is destroyed by the Saxons and they took the kingdom of Powys into their own control."
A later charter depicts a disturbed state of affairs during Ceolwulf's reign: "After the death of Cœnwulf, king of the Mercians, many disagreements and innumerable disputes arose among leading persons of every kind – kings, bishops, and ministers of the churches of God – concerning all manner of secular affairs". In 823, sometime after 26 May, on which date he granted land to Archbishop Wulfred in exchange for a gold and silver vessel, Ceolwulf was overthrown. His replacement was Beornwulf, whose pedigree is not known.
Ceolwulf ruled Kent directly – in his two charters, he is styled as "King of the Mercians and of the men of Kent".
According to a tradition preserved at Evesham, Ceolwulf's daughter Ælfflæd married Wigmund, the son of Wiglaf, King of Mercia 827-839. Wiglaf was succeeded by Beorhtwulf and his son Berhtfrith sought to marry King Wigmund's widow so that he could become king. This was opposed by Wigstan, the son of Wigmund and Ælfflæd, and Berhtfrith slew Wigstan in 849. However, there is no record outside this Evesham tradition that Wigmund was ever king. Alfred Smyth suggests that Ceolwulf II, who became king following the Viking expulsion of King Burgred in 874, may have been another son of Ælfflæd, and thus a grandson of Ceolwulf I.
- Brown, Michelle P.; Farr, Carol A. (1 March 2005). Mercia: An Anglo-Saxon Kingdom in Europe. Bloomsbury Publishing. ISBN 978-1-4411-5353-1.
- Whitehead, Annie (15 September 2018). Mercia: The Rise and Fall of a Kingdom. Amberley Publishing Limited. ISBN 978-1-4456-7653-1.
- Charter S 186, Charter S 187
- Kirby, D. H. (2000). The Earliest English Kings (Revised ed.). London: Routledge. p. 160. ISBN 978-0-415-24211-0.
- Smyth, Alfred P. (1995). King Alfred the Great. Oxford, UK: Oxford University Press. pp. 51–52. ISBN 0-19-822989-5.