The king during the signing of the Magna Carta in England was King John. He ruled from 1199 to 1216 and was known for his unpopular reign due to heavy taxation and disputes with the nobility. The Magna Carta was sealed at Runnymede in 1215 as a response to his overreaching power and the demands of rebellious barons. This charter aimed to limit the king's authority and establish legal rights, contributing significantly to the development of constitutional law. Despite its immediate effects being limited, the Magna Carta's principles laid the groundwork for future democratic developments in England.