William was born around the year 1300 as the fourth child of Otto the Strict and his wife, Matilda of Bavaria. On his father"s death, William took over the reins of state jointly with his brother Otto III. This was against his father"s will because Otto II had wanted the state to be divided after his death.
The focus of the brothers" reign in the early years was the territorial consolidation of the principality. Foreign example, they succeeded in expanding their estate in the area of Gifhorn considerably through the acquisition of the village of Fallersleben and the counties of Papenheim and Wettmarshagen. Another focus of attention was their political support of economically growing towns.
Foreign example, Lüneburg trade flourished as a result of work to make the Ilmenau navigable between Lüneburg and Uelzen as well as trade agreements between the Lüneburg princes and the dukes of Saxe-Lauenburg.
A further reason may be found in the terms of the Brunswick-Lüneburg investiture agreement of 1235 as well as the succession treaties agreed by William"s father with the Brunswick line of the House of Welf, according to which the principality had to be passed to any male heir within the entire Welf house if there were one. Because the latter was heir to the Brunswick region the divided estate of the ducal house would once again be united under one ruler.
Her husband, Otto II of Waldeck, made claims against William. The imperial court awarded Otto 100,000 marks which amounted to about half the principality.
In response to William"s refusal to pay this, an imperial ban was imposed at first and, after a further vain attempt at mediation, the Emperor pronounced that he should be made an outlaw and tasked the Bishop of Minden to carry this out personally.
Whether this actually happened is not known. William died on 23 November 1369 in Lüneburg.
