Siegfried Thalheimer was born in Düsseldorf in 1899, the son of Johanna and Isaak Thalheimer, who was editor-publisher of the Düsseldorfer Lokalzeitung from 1905 until 1931. After obtaining his Ph.D. from the Friedrichs-Wilhelms-Universität in 1926, Thalheimer worked as a journalist for his father's newpaper, writing anti-Nazi articles, as well as art reviews of exhibitions held at two galleries in Düsseldorf, the Galerie Stern and the Galerie Flechtheim. In 1928 he married Gerda Stern, who was born in Mönchengladbach in 1902, the daughter of Julius Stern (1867-1934) and Selma Stern née Heilbron (1871-1964). Before her marriage to Thalheimer, she had trained as a pediatric nurse under Dr. Selma Meyer. Gerda Thalheimer returned to the Luisenschule to complete her Abitur in 1926 and later spent a year and a half studying social sciences and art history in universities at Bonn and Berlin. 

In March 1933, after Thalheimer had been blacklisted for his anti-Nazi activities, he and his wife and daughter Ruth (b. 1929) moved to Saarbrücken, where he founded and edited Westland, an anti-Nazi newspaper that actively opposed the annexation of the Saar to the German state. In 1934 he lost control of Westland and subsequently relocated to Paris, where he published a Jewish political journal, Ordo. When France declared war on Germany in September 1939, he was interned as an enemy alien, released in February 1940, and afterwards served as a prestataire (worker) in the French army. In November 1940, Thalheimer received a United States emergency visa for himself, his wife, and their daughter. The family escaped France via Spain and Morocco and arrived in New York in March 1941. In New York, Thalheimer supported himself and his family by selling paintings provided by his brother-in-law Max Stern that had formerly been part of the inventory of the Galerie Stern, Düsseldorf, and West's Galleries, London. A short time later he formed Fine Arts Associates with Otto Gerson (1902-1962), continuing to sell paintings shipped to New York from Max Stern's Montreal location, the Dominion Gallery. Some of the works that Stern sent to Thalheimer were sold through E. & A. Silberman Galleries, the Rosenberg Gallery, and the Nierendorf Gallery.

Disillusioned with the art business and separated from his wife, Gerda, Thalheimer returned to Germany in the 1950s. He first lived in the Salzburg region, then in Munich, and finally in the Bavarian village of Seenon. During this period in Germany, he resumed his writing career, publishing Gespräch über Deutschlands in 1959 (München, C.H. Beck) and Die Affäre Dreyfus in 1963 (München, Deutscher Taschenbuch Verlag). Siegfried Thalheimer died in Seenon in 1981. Gerda and Ruth remained in New York after Siegfried moved to Germany. Gerda Thalheimer attended Hunter College in New York and Columbia University's library school, where she received a master's degree in 1956. She died in New York in 1994.