William Rosenberg was born in Copenhagen in 1920. He studied at the Royal Danish Academy of Fine Arts in Copenhagen during 1942–43 and again from 1956–58, where his teachers included Aksel Jørgensen, Olaf Rude, and Poul Sørensen. He furthered his artistic education through study trips to Paris, Rome, New York, and Madrid. In 1955, he was appointed set designer at the Royal Danish Theatre, and from 1962 he also worked for Odense Theatre.
Rosenberg made his debut as an actor at the Royal Danish Theatre in 1946 while continuing to pursue his career as a visual artist, working in oil, watercolour, and linocut. He exhibited widely throughout Denmark, including at Charlottenborg, Filosofgangen in Odense, the East Funen Art Association (Østfyns Kunstforening) in Nyborg, Holte Town Hall, and Toldboden in Kerteminde.
His works can be found in several public collections and institutions, including Winter Sun on the Veranda at Odense University Hospital, Leonardo's Last Supper - a free interpretation created for Duborg School in Flensburg - and a copy of Goya's The Second of May 1808.