In 1946, she ran for and was elected mayor of San Juan - the first woman to have been elected mayor of a capital city in the Americas. Under her leadership, San Juan was transformed into a great Latin-American urban center. Rincón de Gautier designed innovative public services and established the first pre-school centers called "Las Escuelas Maternales", which would eventually become the model for the Head Start programs in the United States. She also renovated the public health system and was responsible for the establishment of the School of Medicine in San Juan.Rincón worked together with Ricardo Alegría to restore and conserve the historical structures of Old San Juan and provided housing and basic services to thousands of people. In 1951, during the Cold War era, she ordered the establishment of the island's first Civil Defense system which was under the directorship of Colonel Gilberto José Marxuach. She often opened City Hall to the public and listened to concerns of the residents of the city. In 1959, San Juan was awarded the All American City Award. Rincón de Gautier started a Christmas tradition, which would be continued every year by the governors of Puerto Rico. On the Día de los Reyes (Three Kings Day), celebrated on January 6, she would bring gifts and treats to the poor and needy children. In 1952, 1953 and 1954, she even had plane loads of snow delivered to San Juan so that the children who had never seen or played in snow, would be able to do so.