The origins of Mercia are disputed, but it is generally agreed that the name 'Mercia' (Old English Mierce) means 'borderers, marhlanders' and that the original Mercian territory was somewhere in the Midlands west of the Fens and south of the Humber. The traditional Mercian capital was Tamworth (Staffordshire) but it is not known how far back the association between this site and Mercian royalty goes. As a central territory, Mercia was able to expand into its British hinterland in Wales and Cumbria, as well as coming into conflict with is Germanic neighbours in East Anglia, Northumbria and Wessex. The regnal lists of the Mercians contain many puzzles, but the principal kings are: Offa (A.D. 757-796) (coins only from A.D. 770 onwards); Cynethryth (Queen, wife of Offa); Coenwulf (A.D. 796-821); Ceolwulf I (A.D. 821-823); Beornwulf (A.D. 823-825); Ludica (A.D. 825-827); Wiglaf (A.D. 827-840); Berhtwulf (A.D. 840-852); Burgred (A.D. 852-874); Ceolwulf II (A.D. 874-877). Mercian power was effectively ended by the Danish 'Great Army' of the 860s, although under-kings continued to reign rather than rule for a decade or more.

The most powerful and best known of the Mercian kings, Offa is one of the few early Anglo-Saxon kings whose names are still celebrated today due to his establishment of the border with Wales. Offa rose to power in the Kingdom of Mercia in 757 A.D., after driving out his rival, Beornred; Offa's claim to the throne was based in part on his descent from Pybba, brother of Penda, a 7th century king of the Mercians. Offa was a highly successful ruler and tactician, defeating several other kings to proclaim himself rex totius anglorum 'king of all the English', the first to claim such wide rule. He was probably named for the legendary hero of the Angles, Offa of Angeln, who set out a boundary in single combat against the Swabians at the River Eider in northern Germany. In 779 A.D. he was at war with Cynewulf of Wessex from whom he wrested land in the Thames Valley; ten years later, he secured an alliance with Berhtric of Wessex by giving him his daughter, Eadburg, in marriage. In 794 A.D. he slew Æthelberht of East Anglia, though some accounts ascribe the murder to Cynethryth, Offa's wife. He died after a reign of thirty-nine years and was succeeded by his son, Ecgferth.

Cynethryth was the wife of King Offa, although the date of their marriage is not known. Early in their history, the Anglo-Saxon kingdoms generally shrank from female monarchy and royal ladies were usually regarded as consorts rather than regal personages in their own right. She first witnessed a charter dated 770 A.D., along with cosignatories Ecgfrith and Ælfflæd. By 780 A.D. she was styled Cyneðryð Dei gratia regina Merciorum 'Cynethryth, by the Grace of God, Queen of the Mercians', a unique departure from Anglo-Saxon practice. The imagery employed on her coins follows that used for late Roman empresses, just as that used on Offa's coins depicts him as a late Roman emperor. This coinage is unique in Anglo-Saxon England, and indeed in Western Europe in this period. Cynethryth is associated with her husband in charters and is said to have been a patron of Chertsey Abbey, Surrey. Pope Adrian I, when elevating Higbert's Bishopric of Lichfield to an Archbishopric, wrote to Offa and Cynethryth jointly. Her children with Offa, besides Ecgfrith, included at least four daughters. As an aside, her name is associated with one of the textual cruces in Beowulf (line 1931) where a royal lady named Þryþ is depicted as cruel and wanton until tamed by her future husband, Offa; this is not Offa of Mercia but his ancient ancestor, Offa of Angeln.

King Coenwulf has had a bad press due to the circumstances of his rise to power: he succeeded to the throne on the death of Ecgfrith, the illustrious son of Offa. In 798 he invaded Kent, deposed and imprisoned Eadberht Præn, and made his own brother, Cuthred, king of that territory. Cuthred reigned in Kent from 798 to 807, when he died, whereupon Coenwulf seems to have taken Kent into his own hands. He died in 821, and was succeeded by his brother, Ceolwulf I.

King Coenwulf of Mercia died in Basingwerk in A.D. 821, preparing for another assault on the Welsh territory of Powys. His son, Cenelm, was chosen to succeed him, but was killed soon after, probably fighting the Welsh. There is a tradition of sibling rivalry which ascribes his death to his sister, Cwenthryth. The Mercian throne passed to Coenwulf's brother, Ceolwulf I. The king renewed the assault on the Welsh lands to ther west, and yet found time to meddle in East Anglian affairs over the attempted accession of the prince, Æthelstan. In A.D. 823 the Mercians invaded Powys, but were thwarted by King Cyngen, although they managed to raze the capital of Gwynedd at Degannwy. It was perhapson this campaign that Ceolwulf received his mortal wound, or was ousted by Beornwulf, his successor.

Beornwulf was a descendant of the late King Beornred, and probably represents a rival claimant line within the royal family - this assumption is strengthened by the fact that King Baldred of Kent came to power during his reign while Mercia held sway in the south-east. Baldred's background is not certain, but he was probably a relative of King Beornwulf. The alliteration on B- in this clan implies close ties by marriage and/or consanguinity.

Berhtwulf is known mainly from his coinage and some charters recording land-grants, mostly made at the Mercian royal centre at Tamworth and concerning land in the West Midlands. London - the centre of East Saxon royal power which had been in Mercian hands for about a century - was first successfully attacked by Danes (Vikings) during his reign, and it is probable that territory in the Thames Valley was seized from Mercia by Wessex at this time.

Burgred succeeded Berhtwulf to the Mercian throne in 852, and within a year called upon Æthelwulf of Wessex to aid him in subduing the North Welsh. Their alliance was sealed by Burgred's marriage to Æthelswith, daughter of King Æthelwulf. In 868 the Mercian king appealed to the brothers, Æthelred and Alfred of Wessex, for assistance against the Danes, who were attacking Nottingham. The armies of Wessex and Mercia did little more than harry the invaders, and the Danes were allowed to remain through the winter. In 874 the march of the Danes from Lindsey to Repton drove Burgred from his kingdom and he retired to Rome where he died.

Although his reign was short, Ceolwulf's rulership marked an important departure: the Viking 'Great Army' installed Ceolwulf II as king of Mercia after the abdication of Burgred. Ceolwulf was already a political opponent of Burgred, and the Danes chose to gain control through sponsorship of an Anglo-Saxon ruler rather than by installing one of their own men. Ceolwulf was not content merely to hold a nominal office: he actively pursued an aggressive policy against the neighbouring Welsh states and in 878 he killed Rhodri Mawr of Gwynedd, Powys and Seisyllwg in battle.