Maria Dietrich was born on 28 June 1892, the daughter of the butcher Heinrich Anton Georg Dietrich and Maria Dietrich, née Krach. She stated to Munich local court in 1937 that she had been an art dealer since 1918, and her illegitimate daughter Wilhelmine (Mimi), born in 1910, made a significant contribution to the art shop from the late 1920s. According to her business licence, she first registered in 1921 as a dealer in carpets and antiques at Gabelsbergerstraße 59, Munich. That year she married the Turkish tobacco dealer Ali Almas, who lived in Munich. She took Turkish nationality as a result and converted from Catholicism to Judaism. In February 1933 she left the Jewish community. During her marriage, she and her daughter took the surname Diamant, the German version of Almas. She never officially bore the name Almas, using it only as the name of her art gallery. The couple's divorce in Germany in 1937 was recognized a year later by the Turkish authorities. From that time Maria Dietrich was considered stateless, since her applications to recover her old nationality were rejected. In an interrogation report by the US armed forces in 1945 she claims that she had been questioned repeatedly by the Gestapo on account of her personal situation; no further consequences for her or her daughter are documented. Although she was not politically active and there are no indications of membership of the NSDAP, she donated to the SS and the Nationalsozialistische Volkswohlfahrt (NSV—National Socialist People's Welfare).
Galerie Almas became increasingly prominent in the Munich art market in the early 1930s. It offered a variety of items, including antiques, porcelain, paintings and Persian carpets. Encouraged by the tastes of her main customers, she began to specialize in the nineteenth-century Munich School. Her acquaintanceship with Heinrich Hoffmann gave her access to Hitler, and she sold items to him from 1935. After 1939 she became one of the main buyers for the "Sonderauftrag Linz" (Linz museum project) and provided around 1,000 works for it through her gallery from Germany, France, the Netherlands and Austria. With legitimation from Martin Bormann, from 1938 she was the only German art dealer apart from Ernst Schulte-Strathaus entitled to export art objects from Austria to Germany. In 1938 she purchased above all directly from the collections of persecuted Viennese, concentrating on the works of Ferdinand Georg Waldmüller and Rudolf von Alt, originally owned by Hortense and Hermann Eissler, Gisela and Eduard Schweinburg, Stephan Kerlin, Richard Stein, Irma and Oskar Löwenstein, Edgar Schiffmann, Karl Ruhmann, Felix Stransky, and Margarethe and Julius Buchstab, among others. She made “house calls” and exerted pressure on the owners of the works through verbal threats. When questioned in 1949 by the Monuments, Fine Arts, and Archives Section on her deliveries to Linz, she claimed that all of her transactions in Austria were completed freely. Likely intermediaries included the art dealer Karoline Nehammer, the art historian Bruno Grimschitz and the dealers Maria and Otto Schatzker, with whom she is documented as having been in contact and who had the necessary access to the Vienna art scene. The correspondence between 1939 and 1942 between Bruno Grimschitz and Maria Dietrich indicates that Grimschitz gave her advice and professional appraisals, particularly regarding works by Alt and Waldmüller. As a result, Galerie Almas also made offers to the Österreichische Galerie, but no purchases were made. Apart from her acquisitions from private collections, she was also present between 1938 and 1945 as a buyer at Viennese auctions by Hans W. Lange, in Adolf Weinmüller's Vienna office, at the Dorotheum, and at art dealerships such as the Neue Galerie, Strudlhofgalerie, Galerie Miethke, Galerie St. Lucas and Galerie Neumann.
Although the Bundesdenkmalamt (Federal Monuments Authority) in Vienna wrote in 1949 to the Bundesministerium für Vermögenssicherung und Wirtschaftsplanung (Federal Ministry for Securing Property and Economic Planning) that she had purchased art objects on several occasions in Austria from seized assets, this has been confirmed to date in only two cases (including a Ferdinand Georg Waldmüller family group Old Invalid with Children). A memo from the Ministerium für innere und kulturelle Angelegenheiten (Ministry of Internal and Cultural Affairs) following a visit by Dietrich in 1939 indicates that she was interested in purchasing seized works. Dietrich's daughter Mimi, married name tho Rahde, continued the Munich art dealership again after 1946, specializing in furniture and porcelain. In 1948, the denazification court in Munich judged Maria Dietrich to be non-incriminated. In 1951 she handed over the Munich company to her daughter but remained active until her death in an accident in 1971.
Already in the post-war years, expropriated artworks connected with Galerie Almas were restituted. Since around 2000, further restitutions have taken place from German federal holdings, the Netherlands, France and Austria. For example, in 2009 the Art Restitution Advisory Board recommended the restitution of four chemist's panels by Ferdinand Georg Waldmüller to the successors of Hortense and Hermann Eissler. After the Eisslers had sold the panels to Dietrich in 1938, she passed them on to the "Sonderauftrag Linz". The pictures were held by the Österreichische Galerie following their return to Austria after the war.