Wilhelm was born as the fourth of five children of painter Bernhard Zahn, and his wife, Christiane, née Weis according to church records.

[1] Zahn received lessons in architecture and painting from Andreas Range and Sebastian Weygandt, a portrait painter, in Kassel, Germany.

[2][circular reference] Following Zahn's stay in Paris, beginning in 1824, he spent three years in Italy, particularly in Pompeii, where he made numerous tracings of wall frescoes in the ruins and in the Museo Borbonico (now the National Archaeological Museum, Naples).

He superintended excavations at Herculaneum and Pompeii; cast the finest bronzes and silver vases for the Museum Borbonico; After Zahn's return from Italy in 1827, he met celebrated poet and naturalist Johann Wolfgang von Goethe.

As an architect and interior designer, he decorated many fine houses and villas in the Pompeiian style in England and the United States.