The mayoralty of Fiorello La Guardia lasted from January 1, 1934, to December 31, 1945, while he served as the 99th Mayor of New York City. His mayoralty presided over New York City during the Great Depression and World War II. He is considered the builder of modern New York City due to his numerous infrastructure projects. He replaced John P. O'Brien and was succeeded by William O'Dwyer. During his mayoralty, La Guardia served as president of the United States Conference of Mayors from 1935 until 1945.