Born in New York City of Dutch and Scottish descent, he studied at Harvard. He became leader of the New York legislature in 1884, and president of the New York police board from 1895 to 1897. He was Assistant Secretary of the Navy when in 1898 he raised and commanded the volunteer cavalry known as the 'Roughriders' in the Spanish-American War, returning to serve as Governor of New York State (1898-1900).
Elected Vice-President in 1900, he became President on the assassination of William McKinley (1901), and was re-elected in 1904. During his presidency (1901-09), he strengthened the navy, initiated the construction of the Panama Canal, and introduced a 'Square Deal' policy for enforcing anti-trust laws. His administration acquired the Panama Canal Zone (1903) and began the construction of the Canal, and he received the Nobel Peace Prize in 1906 for his part in the negotiations which ended the Russo-Japanese War.
He returned from a great hunting tour in Central Africa in time to take an active part in the elections of 1910, and created a split in the Republican Party, forming a 'progressive' section with his supporters. As Progressive candidate for the presidency in 1912 he was defeated by Woodrow Wilson. After exploring the Rio Duvida, of Teodoro, in Brazil (1914), he campaigned vigorously during World War I in the cause of US intervention. He wrote on US ideals, ranching, hunting and zoology. He was an immensely popular president; the teddy bear is named after him.