The kingdom was centred on the valley of the River Trent and its tributaries, in the region now known as the English Midlands. The kingdom's "capital" was the town of Tamworth, which was the seat of the Mercian Kings from at least around AD 584, when King Creoda built a fortress at the town.
For 300 years (between AD 600 and 900), having annexed or gained submissions from five of the other six kingdoms of the Heptarchy (East Anglia, Essex, Kent, Sussex and Wessex), Mercia dominated England south of the River Humber: this period is known as the Mercian Supremacy. The reign of King Offa, who is best remembered for his Dyke that designated the boundary between Mercia and the Welsh kingdoms, is sometimes known as the "Golden Age of Mercia". Nicholas Brooks noted that "the Mercians stand out as by far the most successful of the various early Anglo-Saxon peoples until the later ninth century", and some historians, such as Sir Frank Stenton, believe the unification of England south of the Humber estuary was achieved during the reign of Offa.
Mercia was originally a pagan kingdom, but King Peada converted to Christianity around 656, and Christianity was firmly established in the kingdom by the late 7th century. The Diocese of Mercia was founded in 656, with the first bishop, Diuma, based at Repton. After only 13 years at Repton, in 669 the fifth bishop, Saint Chad, moved the bishopric to Lichfield, where it has been based ever since. In 691, the Diocese of Mercia became the Diocese of Lichfield. For a brief period between 787 and 799 the diocese was an archbishopric, although it was officially dissolved in 803. The current bishop, Michael Ipgrave, is the 99th since the diocese was established.
At the end of the 9th century, following the invasions of the Vikings and their Great Heathen Army, much of the former Mercian territory was absorbed into the Danelaw. At its height, the Danelaw included London, all of East Anglia and most of the North of England.
The final Mercian king, Ceolwulf II, died in 879; the kingdom appears to have thereby lost its political independence. Initially, it was ruled by a lord or ealdorman under the overlordship of Alfred the Great, who styled himself "King of the Anglo-Saxons". The kingdom had a brief period of independence in the mid-10th century, and again very briefly in 1016; however, by this time, it was viewed as a province within the Kingdom of England, not an independent kingdom.
Mercia is still used as a geographic designation, and the name is used by wide range of organisations, including military units, public, commercial and voluntary bodies.