Roosevelt served as assistant secretary of the navy and governor of New York before winning election as William McKinley 's running mate in the 1900 presidential election . Roosevelt became president following the assassination of McKinley by anarchist Leon Czolgosz in Buffalo , New York ; Czolgosz shot McKinley on September 6 , 1901 , and McKinley died on September 14 . Roosevelt was sworn into office on the day of McKinley 's death at the Ansley Wilcox House in Buffalo . John R. Hazel , U.S. District Judge for the Western District of New York , administered the oath of office . Being just a few weeks short of his 43rd birthday , Roosevelt became the youngest president in U.S. history , a distinction he still retains
assassination of McKinley
Assassination of William McKinley
Presidency of Theodore Roosevelt