The earliest written reference to the town is in the Anglo-Saxon Chronicle, which records that in 733 the King of Wessex, Æthelheard lost control of Somerton to Æthelbald, King of Mercia. Somerton was the site of the 949 meeting of the witan, a form of Anglo-Saxon parliament. The town returned to West Saxon royal control in the ninth century, and it was listed in the Domesday Book of 1086 as "Sumertone". The name may come from Old English for "sea-lake enclosure", "summer town" or "summer farmstead". The Somerton name was extended to the people in the area it controlled, and this area became known as Somerset, although Somerton soon ceased to be the most important settlement and never grew into a large town. The parish was the largest in the Hundred of Somerton. It was, briefly, the county town of Somerset from the late thirteenth century into the early fourteenth century. A building referred to as "Somerset castle" is believed to have been built around 1280 as a county gaol, with a visitor in 1579 describing the remaining portion as "an old tower embattled about castle-like". It was owned by Sir Ralph Cromwell between 1423 and 1433. Details are vague and visible remains have vanished, so its status as a castle and its very existence is in doubt, with one writer, D.J.C. King, feeling that people were confusing it with Somerton Castle in Lincolnshire.
