William Augustus was regarded as extremely learned and, like his father, as a follower of the Lutheran doctrine.

In 1582, he undertook a Grand Tour to France and England and then enrolled with his brothers at the University of Helmstedt.

After the death of his father, he took up the government of Harburg jointly with his brothers Christopher and Otto III.

After the death of the Duke Frederick Ulrich of Brunswick-Wolfenbüttel, he received the County of Hoya from his inheritance.

His inheritance was divided by Dukes of Frederick IV of Brunswick-Celle and Augustus II of Brunswick-Wolfenbüttel.