Marianne Schmidl was the daughter of Josef Schmidl (1852–1916) and Maria Schmidl née Friedmann (1858–1934). A Viennese lawyer, Josef was born Jewish but converted to Christianity in 1889; Maria was Christian. Maria was the granddaughter of the artist Friedrich Olivier and Franziska Heller; Franziska's sister was married to the artist Julius Schnorr von Carolsfeld. Thus Marianne Schmidl was Olivier's great-granddaughter, and the great-grandniece of Julius Schnorr von Carolsfeld.
Marianne Schmidl grew up in the lively cultural environment of prosperous, middle class, fin de siècle Vienna. She appears to have inherited her mother's strength of intellect and character. In 1910, despite the prejudices against women being admitted to university, Marianne enrolled to study physics and mathematics. In 1913 she switched courses to anthropology and ethnology and developed what was to become a lifelong fascination with Africa. Her research was based on the extensive collections of German and Austrian ethnological museums and culminated in the 1915 publication of her doctoral thesis "Numbers and Counting in Africa," making her the first woman to receive a doctorate in ethnology from the University of Vienna.
After World War I, Marianne Schmidl worked at the Museum für Völkerkunde in Berlin where she began to research African basketry. She worked at the Linden Museum in Stuttgart for three years and thereafter at the Nationalbibliothek in Vienna. In addition to her work at the library, from 1926 on she contributed to a research project on African handicrafts for the Museum für Völkerkunde in Vienna, financed by the Sächsische Forschungsinstitut für Völkerkunde in Leipzig.
In January 1938 Marianne Schmidl was promoted to librarian first class at the Nationalbibliothek. In February 1938 she was granted a six-month medical leave. With the German annexation of Austria in March 1938, public employees were required to submit proof of their Aryan descent. On June 25, 1938, Marianne was informed that at the conclusion of her medical leave she would no longer be allowed to work at the library because of her Jewish heritage.
On September 30, 1938, Marianne Schmidl submitted the required registration of her assets—Vermögensanmeldung—on which she listed the value of her "metal, jewelry, luxury items, art and collections." In October she submitted the estimated value of her half of the house she shared with her brother-in-law, Dr. Karl Wolf, husband of her deceased sister Franziska (1891–1925), for which she would be taxed. Throughout 1938 and 1939 Schmidl fought through official channels the various Jewish taxes and her loss of work. Schmidl was by now completely dependent on the financial support of Dr. Wolf. As a non-Jew, Wolf managed to help her keep the family home; however, with no income she had to sell the family collection of Olivier and Schnorr von Carolsfeld drawings.
In the spring of 1942 Marianne Schmidl was deported to Izbica, Poland, along with more than 5,000 Viennese Jews. Izbica was a holding point for the concentration camp Belzec. Schmidl's family received only three messages from her by May of that year, after which she was not heard from again. In July 1942, the German government expropriated her half of the house she shared with Dr. Wolf. She was declared dead in May 1950.