High Quality Content by WIKIPEDIA articles! The Kingdom of Mercia was an important state in the English Midlands from the 6th century to the 10th. For some two hundred years from the mid-7th century onwards it was the dominant member of the Heptarchy and consequently the most powerful of the Anglo-Saxon kingdoms. During this period its rulers became the very first English monarchs to assume such wide ranging titles as King of Britain and King of the English.