His parents were Heinrich Rosengart (1850–1921), a bed feather maker, and Selma Thannhauser.
[1] In 1912 Rosengart was influenced by a visit to the Sonderbund exhibition in Cologne in 1912, which featured modern art.
[1] Rosengart sold artworks to museums, like the Kunstsammlung Nordrhein-Westfalen (North Rhine-Westphalia Art Collection) in Düsseldorf, as well as to private collectors like Etta and Claribel Cone (Gauguin, "Woman with Mango," Matisse, "Odalisque," Picasso, "Mother and Child," now Baltimore Musée).
The core of the collection consists of 47 paintings and drawings by Picasso from the years between 1904 and 1972, as well as 125 selected Klee works from all creative periods, making it one of the most important private Klee collections internationally.
It also includes three paintings by Cezanne as well as paintings by Monet, Pissarro, Renoir, Seurat, the Nabis artists Bonnard and Vuillard, and works by Modigliani, Soutine, Matisse, Braque, Léger, Kandinsky, Miró and Chagall.