In 1898, Margarethe Herz married Siegmund von Sonnenthal, son of the well-known actor Adolf von Sonnenthal, in the Vienna City Temple. He had joined Siemens & Halske Aktiengesellschaft as a chemist in April 1892 and remained with the company, which had since merged with Österreichische Schuckert-Werke, as an authorised signatory until 1904. He later served as director of the Cartel of Cable Works of the Austro-Hungarian Monarchy, which was founded in autumn 1904. At the same time, he was also very interested in culture: He was in contact with the art historian and long-time director of the Picture Gallery of the Kunsthistorisches Museum Gustav Glück and the writer, patron and art collector Karl Lanckoroński, among others. Together with her husband and two daughters, Eva Henriette and Gertrud Eleonore, Margarethe Sonnenthal lived at Anton-Frank-Gasse 20 in Vienna-Währing. On 24 May 1927, just a few weeks before the death of their younger daughter Gertrud, the family left the Jewish Community of Vienna and converted to Catholicism. In 1931, Siegmund Sonnenthal donated works on loan to the Bundestheatermuseum, which was housed in the Burgtheater, in memory of his father. When he died two years later, his widow donated a partial estate of around 14,000 individual items, including role books, playbills and portrait photos, to the theatre collection, which at the time was still part of the National Library. These objects are still in the Theatermuseum today. Shortly after the annexation, on 14 March 1938, Margarethe Sonnenthal asked the director of the theatre collection and the Federal Theatre Museum, Joseph Gregor, for the return of her deceased husband's loans, which actually took place quickly, probably due to the dissolution of the museum in the Burgtheater in March 1938. In order to escape Nazi repression and persecution, Margarethe Sonnenthal decided to move to Upper Bavaria in April 1941, where she lived incognito in Alzing. When her identity was discovered, Margarethe Sonnenthal committed suicide in March 1942. Her daughter Eva Henriette Sonnenthal had emigrated to London in August 1939, where she died in a retirement home in 1973.

Of the artist memorabilia that had been returned to Margarethe Sonnenthal in 1938, some was returned to the Bundestheatermuseum during the Nazi era via unclear channels and later became part of the theatre collection and today's Theatermuseum. These included a watch chain, a miniature medal chain and a laurel wreath. In October 2008, the Art Restitution Advisory Board issued a recommendation for restitution. In December 2016, an updated dossier was presented to the Art Restitution Advisory Board, which took note of the change - an ebony box from the Sonntenthal property had been identified - and approved the original recommendation. The Theatermuseum repurchased the objects after the restitution in 2017.