In 1953 Dwight Eisenhower appointed her as the first United States Ambassador to Switzerland; she served in that role until 1957. While serving as ambassador she was the only woman to attend the Big Four Summit Conference in Geneva. Her appointment caused some unease to the thoroughly conservative Swiss government, which struggled to find a proper form of address for a "Fräulein" ambassador. "The New York Times" covered the appointment under the title "Slap at Anti-Feminist Swiss", linking it to the Swiss refusal to allow women to vote. Even more diplomatic headaches were caused by Willis' frank anti-Communist rhetoric, which enraged the Swiss left-wing press. The Swiss government asked the national press agency to give less coverage to her public appearances.