Mercia's exact evolution from the Anglo-Saxon invasions is more obscure than Northumbria, Kent, or even Wessex. Archeological surveys show that Angles settled the lands north of the Thames river by the sixth century. The name Mercia is Old English for "boundary folk", and the traditional interpretation was that the kingdom originated along the frontier between the Welsh and the Anglo-Saxon invaders, although P. Hunter Blair has argued an alternative interpretation that they emerged along the frontier between the kingdom of Northumbria and the inhabitants of the Trent river valley.
While the earliest known king of Mercia was named Cearl, and is only known because he gave his daughter in marriage to Edwin, king of Deira, the first Mercian king we know more about than his name is Penda, who ruled c.632 - 654. The little that is known about Penda is through the hostile account of Bede, who disliked him both for being an enemy king to Bede's own Northumbria, but also for being a pagan. After a reign of successful battles against all opponents, Penda was defeated and killed at the Battle of Winwaed by the Northumbrian king Oswiu in 654.
Penda was succeeded first by his son Peada, but in the spring of 656 Oswiu assumed control of the whole of Mercia. A revolt in 657 resulted with the appearance of another son of Penda, Wulfhere, who ruled Mercia until he was defeated and killed in an invasion of Northumbria in 674. The next two kings, Aethelred and Cenred son of Wulfhere, are better known for their religious activities. And the king who succeeded them, Ceolred, is said in a letter of Saint Boniface to have been a dissolute youth who died insane. So ended the rule of the direct descendants of Penda.
At some point before the accession of Æthelbald, the Mercians conquered the region around Wroxeter, known to the Welsh as "The Paradise of Powys." Elegies written in the persona of its dispossed rulers record the sorrow at this serious dispossession.
The next important king of Mercia was Æthelbald (716 - 757). For the first few years of his reign, he had to face the obstacles of two strong rival kings, Wihtred of Kent and Ine of Wessex. But when Wihtred died in 725, and Ine abdicated his throne the following year to become a monk in Rome, Æthelbald was free to establish Mercia's hegemony over the rest of the Anglo-Saxons south of the Humber. Because of his prowess as a military leader, he acquired the title of Bretwalda.
Following the murder of Æthelbald by one of his bodyguard, a civil war followed, which was concluded with the victory of Offa. Offa was forced to build the hegemony over the southern English of his predecessor anew, but he not only did so successfully, he became the greatest king Mercia ever knew. Not only did he win battles and acquire the title of Bretwalda, he also took an active hand to administering the affairs of his kingdom by founding market towns and overseeing the first major issues of coins in Britain, assumed a role in the administration of the Catholic church in England, and even negotiated with Charlemagne as an equal.
Offa even exerted himself to ensure that his son Ecgfrith would succeed him when he died on July 26, 796. But his efforts were in vain: Ecgfrith ruled for less than five months, and the kingdom passed to a distant relative named Cenwulf (796 - 821). Cenwulf himself was succeeded by his brother Ceolwulf, who demonstrated his military prowess by his attack and destruction on the fortress of Deganwy in Powys. This did not dissuade Egbert (802 - 839), king of Wessex, from revolting from his overlordship, and Egbert defeated his successor Beornwulf at Ellendun in 825.
The Battle of Ellendun proved decisive. Beornwulf was slain suppressing a revolt amongst the East Angles, and his successor (a former ealdormen Ludeca) held the fraying strands of Mercia together for only four more years until Egbert conquered Mercia. Mercia soon returned to the rule of her own king, but its days as the leading power of England had passed. In 874, Danish armies invaded the region, and in 886, the eastern part of the kingdom became part of the Danelaw, while the western portion was occupied by Wessex.
For knowledge of the internal composition of the kingdom of Mercia, we must rely on a document of uncertain age, the Tribal Hidage. This lists a number of people who eventually vanished, except for reminders in various placenames.