Hiltgunt Margret Zassenhaus was born in Hamburg, Germany. Her father Julius Zassenhaus was a liberal history professor who lost his job in 1933 when the Nazi regime came to power. Her mother Margret Ziegler Zassenhaus was an anti-Nazi dissident who helped smuggle Jews out of Germany. Hiltgunt studied philology, specializing in Scandinavian languages, and graduated from the University of Hamburg in 1939. She continued her language studies at the University of Copenhagen until the outbreak of World War II. In 1940, she was hired to censor letters from German Jews in the ghettos. Instead, she sometimes smuggled the letters out or added messages in the margins. She resigned that job in 1942 and entered medical school in Hamburg. At that time, she was asked by the Third Reich's Justice Department to monitor pastoral visits and letters to and from Danish and Norwegian resistance prisoners. She gained the trust of the prisoners, and managed to bring them news of the war and messages from their families, and smuggle in food, medicine, and writing supplies. When World War II was drawing to a close, she learned of a plan to execute all political prisoners, and passed on the information to the Red Cross. As a result, 1,200 Scandinavian prisoners were freed and transported out of Germany.
After the war, she was unable to complete her medical studies due to the damage inflicted Hamburg. She arranged to smuggle herself into Denmark in 1947; later the Danish Parliament passed a special law to legitimize her status. She continued her medical studies at the University of Bergen, and finally graduated as a physician from the University of Copenhagen. She emigrated to the USA in 1952, settling in Baltimore, Maryland. She completed her internship and residency at City Hospital and opened her own medical office in 1954 as H. Margret Zassenhaus. She wrote two books about her wartime experiences, Halt Wacht im Dunkel (On Guard in the Dark, 1947) and Walls: Resisting the Third Reich (1974). In 1978, she was featured in a British television series called Women in Courage. She and some of the surviving prisoners also were the subjects of a 1980 British television documentary, It Mattered to Me. She was honored by several nations for her lifesaving efforts in World War II and received the Royal Norwegian Order of St. Olav, the Red Cross Medal, the Danish Order of the Dannebrog, and the British Order of Merit, among others.