William was the fifth and youngest son of Henry the Lion and Matilda, the eldest daughter of King Henry II of England. He was born in Winchester, England during his father"s exile. He probably remained there when Henry returned to Saxony and was raised at King Richard"s court.
After his unsuccessful uprising, Henry had submitted to the Hohenstaufen emperor Frederick Barbarossa in 1181 and though he had to leave Germany, he could retain the Welf possessions around Lüneburg, Braunschweig, and Haldensleben. William was extradited to Duke Leopold V of Austria and temporarily held in Hungary. The Welf brothers entered into an agreement with the Cologne archbishop Adolf of Altena, who in 1198 crowned Otto King of the Romans during the throne quarrel with the Hohenstaufen heir Philip of Swabia.
Both entered into matrimony in spring 1202, accompanied by the provision of a significant dowry.
Their only child was Otto (1204–1252), who inherited his father"s property and became the first Duke of Brunswick-Lüneburg in 1235. William"s hopes to assume the rule in Holstein, however, were disappointed by the Danish court.
In May 1202, the Welf brothers met at Paderborn, where they divided their father"s heritage. William received the northern territories up to the Danish border around the town of Lüneburg, the territory of Lauenburg beyond the Elbe River, Hitzacker, Lüchow, and Dannenberg, as well as the lands around Haldensleben and in the eastern Harz mountain range including Blankenburg and Heimburg with Regenstein Castle.
William concentrated on consolidating his rule, strongly relying on the salt trade around Lüneburg, which became his permanent residence.
