Diegelmann's first stage appearance was in 1878 in the chorus for the Frankfurt Opera. In 1881 he debuted at the Frankfurt City Theater, playing King Lear, William Tell, and other title characters. Sometime around 1900, Diegelmann relocated to Berlin. Here, he appeared at a variety of theaters, including the Deutsches Theater, the Großes Schauspielhaus, and the Deutsches Künstlertheater. Diegelmann was introduced to film in 1913 by Max Reinhardt. he became a prolific actor of supporting roles, often as a father figure. He performed in the well-known film Der Blaue Engel (The Blue Angel), where he played a ship captain who flirts with the lead actress (Marlene Dietrich). The final film he starred in was the Theodor Storm adaptation of Der Schimmelreiter (The Rider on the White Horse). He continued to make stage appearances after this, with his last role being in Rembrandt vor Gericht. He died not long after this. He was laid to rest at the Stahnsdorf South-Western Cemetery near Berlin, though the grave itself no longer exists.^[1][2]

• Kosta, Barbara. Willing Seduction: The Blue Angel, Marlene Dietrich, and Mass Culture. Berghahn Books, 2009.

• Ludwig Eisenberg's großes biographisches Lexikon der deutschen Bühne im XIX. Jahrhundert. List, Leipzig 1903, S. 198.
• Kay Weniger: Das große Personenlexikon des Films. Die Schauspieler, Regisseure, Kameraleute, Produzenten, Komponisten, Drehbuchautoren, Filmarchitekten, Ausstatter, Kostümbildner, Cutter, Tontechniker, Maskenbildner und Special Effects Designer des 20. Jahrhunderts. Band 2: C – F : John Paddy Carstairs – Peter Fitz. Schwarzkopf & Schwarzkopf, Berlin 2001, ISBN 3-89602-340-3, S. 386–387.

• Wilhelm Diegelmann at IMDb