Justin K. Thannhauser was the son of Heinrich Thannhauser (1859–1935), who was also an art dealer. As a child, Justin assisted his father in his Moderne Galerie, one of the three Thannhauser Galleries which was located in Munich. In 1911, Heinrich sent him abroad to further his academic studies, including art history, philosophy, and psychology. Justin studied in Munich, Berlin, Florence, and Paris, with renowned scholars such as Henri Bergson, Adolf Goldschmidt, and Heinrich Wölfflin. He later brought Wölfflin and other eminent guests to hold private lectures at the Moderne Galerie, helping to turn it into one of Munich's leading art galleries. While abroad, he also strengthened business contacts with artists and other important art dealers, such as Kahnweiler (Pablo Picasso's Parisian dealer) and Wilhelm Uhde.
In 1912, Justin returned to Munich to work full-time at the gallery, which had become known for its combination of Munich Modernism (see Neue Künstlervereinigung München and Der Blaue Reiter) and French avant-garde painting (see Impressionism and Post-Impressionism). His new business relationship with Filippo Tommaso Marinetti, however, led to an unprecedented exhibition of works by the Italian Futurists. This exhibition, while controversial, brought even greater notoriety to the gallery. In 1913, it held the largest and most comprehensive exhibition of Picasso's work. Justin wrote the introduction to the exhibition catalogue and soon began a close personal and professional relationship with the artist. He acquired many of Picasso's works over the years, including Woman Ironing (1904)  and The Blind Man's Meal (1903).
World War I and the interwar period
Justin was called to serve in World War I in 1914. After being wounded in 1916, he returned to Munich and helped Heinrich to publish three large volumes cataloguing a selection of the gallery's inventory and activities. In 1918, he married his first wife, Kate. She gave birth to their two sons, Heinz and Michel, in 1918 and 1920, respectively. In 1919, as the political and economic situation in Germany continued to decline, Justin decided to move his family to Lucerne, Switzerland. There, he opened a second branch of what was now called Moderne Galerie/Thannhauser. This new venue allowed the family to sell works that were found unfavorable in Germany. Justin ran the Lucerne branch until 1921, when he was called back to Munich to assist his father, who had developed a serious condition in his larynx. The Lucerne gallery continued to be under Justin's direction until 1928, when his cousin Siegfried Rosengart assumed control and changed its name to Galerie Rosengart.
Once in Munich, Justin assumed complete control of his father's gallery and brought the two branches under the name Galerien Thannhauser. He began to slowly rebuild the business's reputation, which had weakened during the war, by organizing conservative exhibitions of German paintings and works on paper. He soon returned to the avant-garde, however, showing works by Picasso and Kandinsky in 1922, an exhibition of contemporary American artists in 1923, and paintings by Vlaminck in 1925. His most daring endeavors took place in 1926, when he held exhibitions of work by George Grosz and Otto Dix.
In 1927, Justin opened a third gallery in Berlin. The success of this branch quickly surpassed that of the one in Munich; he thus decided to focus completely on the former and closed the latter in 1928. His greatest achievement in the Berlin space took place in 1930, when he presented the largest exhibition of works by Matisse ever held in Germany.
World War II and post-war years
In 1937, Justin moved with this family to Paris to escape the Nazi Germany regime. Although the Nazi government considered Modern art to be "degenerate," he had paid a steep export tax and was thus permitted to bring many important works and archive materials with him. He was, however, forced to liquidate his family's collection of classic German art in order to make this financially possible. Despite this considerable loss, he was able to open a private gallery on rue de Miromesnil. He was also voted into the Syndicat des Editeurs d'Art et Négotiants en Tableaux Modernes, Paris's professional society of art dealers.
After the outbreak of World War II, Justin moved his family back to Switzerland, and then to New York. There, he opened another private gallery. Fortunately, he had managed to bring several important works with him to the United States; the house in Paris would later be looted by Nazi soldiers. However, due to the death of Heinz (who was killed in combat in 1944) and the poor health of Michel (who would ultimately die in 1952), Justin canceled his plans to open a public gallery and placed a large number of works up for auction in 1945. He remained in New York until 1971, where he continued to operate the private gallery, collected art, and assisted museums and galleries with exhibitions and acquisitions. He also hosted many international cultural luminaries in his home, including Picasso, Louise Bourgeois, Henri Cartier-Bresson, Marcel Duchamp, Jean Renoir, John D. Rockefeller, and Thomas M. Messer, then Director of the Solomon R. Guggenheim Museum, among many others. Kate died in 1960, and he married his second wife, Hilde, two years later.
In 1963, Justin decided to bequeath the essential works of his collection to the Guggenheim. The size and quality of this gift was unparalleled by any that he had made or would make again. Because the terms called for the works to be permanently installed in a designated space so that they would be publicly accessible, the Guggenheim created the Thannhauser Wing in 1965. The museum space housing the Thannhauser collection has since been expanded and restored.
Justin and Hilde retired to Switzerland in 1971, and Justin died in Gstaad on December 26, 1976. After his death, Hilde continued to donate works of art to institutions around the world, including the Kunstmuseum Bern and the Guggenheim. She died in Bern in 1991.
- "Exhibitions: Thannhauser Collection". Solomon R. Guggenheim Museum. Archived from the original on 25 June 2014. Retrieved 16 June 2014.
- "Collections: Thannhauser Collection". Solomon R. Guggenheim Museum. Retrieved 18 June 2014.
- Pablo Picasso's Woman Ironing, The Solomon R. Guggenheim Museum
- The Blind Man's Meal, The Metropolitan Museum of Art
- "Collection Online: Major Acquisition > Thannhauser Collection". Solomon R. Guggenheim Museum. Retrieved 18 June 2014.
- Bäckström, Per; Hjartarson, Benedikt, eds. (2014). Decentering the Avant-Garde, Volume 30 of Avant-Garde Critical Studies. Rodopi. p. 189. ISBN 978-9-401-21037-9.
- Barnett, Vivian Endicott (1978). The Guggenheim Museum, Justin K. Thannhauser Collection. Solomon R. Guggenheim Museum. p. 14. ISBN 978-0-892-07016-9.
- Drutt, Matthew: "A Showcase for Modern Art: The Thannhauser Collection," in: Drutt, Matthew (Ed.), Thannhauser: The Thannhauser Collection of the Guggenheim Museum, The Solomon R. Guggenheim Foundation 2001, pp. 1–25
- The Thannhauser Gallery: Marketing Van Gogh, Van Gogh Museum, Amsterdam, 2017. [book review in: http://www.cosmopolis.ch/english/art/e0021000/thannhauser_gallery_e0210000.htm]