Archduke Wilhelm was the youngest son of Archduke Karl Stephan and Archduchess Maria Theresia, Princess of Tuscany. He was born in a family estate on the Lošinj island, Austrian Littoral (present day Croatia). During his arrest by the Soviet authorities he indicated the city of Pola (at the time of arrest in Italy, today Pula, Croatia). Wilhelm also indicated that his social class is "of landowners" (, pomeschik; Lord of the manor) and he has no occupation. The last one was not accurate as at the moment of arrest he was a businessman-entrepreneur.