Florence Mendheim (1899-1984) was the daughter of German-Jewish immigrants had moved to the United States in the 1880s and still had close contact with their family back in Berlin. She worked as a librarian in the Washington Heights branch of the New York Public Library for 25 years, from 1919 to 1944. In 1933, she learned that Rabbi J.X. Cohen of the American Jewish Committee wanted volunteers who were not obviously Jewish to infiltrate the growing American Nazi movement, she did not hesitate. Cohen provided her with a fake address, a fake name, and a Nazi party pin. For the next five to six years, she spent her evenings spying on American Nazis.
Under the pseudonym Gertrude Mueller, she attended meetings and rallies of the pro-Nazi group Friends of New Germany and its successor organization, German American Bund. She gathered names, took notes as a secretary for the groups, and accumulated pro-Nazi and antisemitic propaganda and literature. Sometimes at the end of the meeting, another participant would offer to drive her home, making that fake address valuable indeed. It was terrifying. She could never be sure whether the offer was made as an act of politeness or because someone suspected her.
She used two other pseudonyms as well as Gertrude Mueller. She signed her reports to the American Jewish Committee with the random initials KQX. She used the name Anna Hitler as a cover for doing research on Hitler at various academic institutes under the guise of doing genealogical research.
Mendheim appears to have quit spying in 1938 or 1939. The need for such work ended several years later. When United States entered the war at the end of 1941, they cracked down on pro-Nazi organizations, perhaps with help of reports from Florence Mendheim and others who had been brave enough to spy on pro-Nazi meetings.