He was born in Dessau, the eldest son of Leopold I, Prince of Anhalt-Dessau, by his morganatic wife Anna Louise Föhse.
In 1706, the eight-year-old William Gustav was appointed a captain and accompanied his father in his campaign against France in 1712.
They had nine children:[1][better source needed][2] In 1737, during Johanna Sophia's ninth and last pregnancy, William Gustav came down with smallpox and, wanting to see her before his death, he had her and his eldest son brought to Dessau, revealed the secret marriage and children to his father, and entrusted their care to him.
Prince Leopold raised the eldest son at his court and gave a secret pension to the widow and her other children in Kleckewitz.
[3] At the same time, King Frederick II of Prussia raised the two illegitimate sons whom William Gustav sired with one "Henriette Marianne Schardius" to the rank of nobility with the surname "of Anhalt":