Born into a wealthy family in New York City, Roosevelt was a sickly child who suffered from asthma and stayed at home studying natural history. To compensate for his physical weakness, he embraced a strenuous life. Home-schooled, he became an eager student of nature.

He attended Harvard University, where he studied biology, boxed and developed an interest in naval affairs. In 1881, one year out of Harvard, he was elected to the New York State Assembly, where he became a leader of the reform faction of his Republican Party (the "GOP"). His book The Naval War of 1812 (1882) established his professional reputation as a serious historian; he wrote numerous books on hunting, the outdoors, and current political issues, as well as frontier history.

In 1884, his wife and his mother died on the same day. He left politics and went to the frontier, becoming a rancher in the "Badlands" in the Dakotas. Returning to New York City, he ran for mayor in 1886, finishing third with 60,000 votes.

Appointed (1889) by President Benjamin Harrison as a member of the Civil Service Commission, he was noted for his vigor in the post until he resigned in 1895. As head (1895-97) of the New York City police board, Roosevelt gained public notice by his advocacy of reform.

In 1897 he returned to federal office as Assistant Secretary of the Navy under President McKinley. An ardent supporter of U.S. expansion, he worked toward putting the U.S. navy on a war basis for the coming war with Spain. The Spanish-American War broke out in 1898 while Roosevelt was, effectively, running the Department of the Navy. He promptly resigned to organize the 1st U.S. Volunteer Cavalry, known as the Rough Riders. As their commander, he led them into battle at Kettle Hill near Santiago de Cuba, which newspaper accounts called the “charge up San Juan Hill.”

Roosevelt's battlefield exploits, recounted in his book The Rough Riders (1899), won him the governorship of New York. His reform program so upset party leaders that they arranged for him to receive the Vice Presidential nomination in 1900. Roosevelt was so bored with the inactivity of the Vice Presidency that he seriously considered finishing law school. But six months after his inauguration, on September 14, 1901, President William McKinley died of an assassin's bullet and Roosevelt took the Presidential oath.

The McKinley-Roosevelt slate was elected, but Roosevelt served as Vice President only a few months. McKinley was assassinated, and Roosevelt became (Sept. 14, 1901) President shortly before his 43d birthday, making him the youngest person to hold that office.

At 42, Roosevelt was the youngest person ever to assume the office. He pledged to continue McKinley's policies but soon demonstrated his reformist and independent characteristics.

Roosevelt instituted more than 30 court cases against corporations, charging them with violations of antitrust laws—conspiring to control markets or fix prices. He insisted that coal mine owners negotiate with their miners. This was the first time that a President had acted as a neutral umpire in a dispute between management and labor.

In 1902 he secured passage of the Newlands Reclamation Act, which funded irrigation projects in the West. He increased the acreage of national parks and forests fivefold, much of it by executive orders creating five national parks. He also established the first federal bird reservation and 50 bird sanctuaries to protect endangered species. For the first time, a President focused public attention on conservation and the environment, and he got Congress to establish the U.S. Forest Service.

In 1903 Congress created the Department of Commerce and Labor, and to head it, Roosevelt nominated the first Jewish cabinet secretary, Oscar Straus.

In foreign affairs Roosevelt presided over the expansion of American naval power, sending the Great White Fleet on a tour around the world from 1907 to 1909 to demonstrate the power of the United States to other nations. He insisted that the United States be the dominant naval power in the Pacific.

When the government of Colombia refused to ratify an agreement that would allow the United States to begin construction of a canal across the isthmus of Panama (then a Colombian province), Roosevelt encouraged revolutionists to declare Panama independent and used the navy to prevent Colombian warships from quelling the revolt. Soon, he concluded an agreement with the new nation, granting the United States a zone in which to construct a canal.

In 1904 the President announced the Roosevelt Corollary to the Monroe Doctrine, which, in effect, made the United States the “policeman” in the Western Hemisphere.

In 1904 Roosevelt was unanimously nominated for President by the Republican party, and he won election in his own right by defeating Democrat Alton B. Parker. He declared that he viewed his first three years in the White House as a full first term, and that, therefore, his second term would be his last.

Roosevelt continued his activist foreign policies. He took full control of the finances of the Dominican Republic in 1905 in order to pay its debts to U.S. and European creditors.

That same year Roosevelt mediated an end to the Russo-Japanese War at the Portsmouth Conference, receiving the 1906 Nobel Peace Prize for his efforts. He donated his $40,000 prize to a foundation for promoting better labor-management relations.

With solid Republican majorities in both chambers of Congress, in 1906 Roosevelt won passage of three important laws: the Pure Food and Drug Act and the Meat Inspection Act, which established new safety standards for consumers, and the Hepburn Railroad Act, which strengthened the enforcement power of the Interstate Commerce Commission over railroads.

Building on McKinley's effective use of the press, Roosevelt made the White House the center of news every day, providing interviews and photo opportunities. After noticing the White House reporters huddled outside in the rain one day, he gave them their own room inside, effectively inventing the presidential press briefing. The grateful press, with unprecedented access to the White House, rewarded Roosevelt with ample coverage.

In 1908 Roosevelt honored his two-term pledge and helped secure the Republican Presidential nomination for his protégé William Howard Taft.

In March 1909, shortly after the end of his presidency, Roosevelt left New York for a safari in east and central Africa. Roosevelt and his companions killed or trapped approximately 11,400 animals, from insects and moles to hippopotamuses and elephants, including 512 big game animals, including six rare white rhinos.

In 1908 Roosevelt had helped secure the Republican Presidential nomination for his protégé William Howard Taft. The two men later broke over Taft's conservative policies, and in 1910 Roosevelt went on a nationwide speaking tour, promoting a program of New Nationalism. He called for government regulation of corporations and natural resources, a minimum wage, and limitations on the length of the workday.

In 1912 Taft defeated Roosevelt for the Republican nomination. Roosevelt's followers then organized a new party, the Progressive party, and nominated him. Roosevelt told them he felt “as strong as a bull moose,” and the press then dubbed it the Bull Moose party.

With Republican voters split between the regular Taft and the insurgent Roosevelt, Democrat Woodrow Wilson won the White House.

While Roosevelt was campaigning in Milwaukee, Wisconsin, on October 14, 1912, a saloonkeeper named John Schrank shot him, but the bullet lodged in his chest only after penetrating his steel eyeglass case and passing through a thick (50 pages) copy of his speech. Roosevelt, as an experienced hunter, correctly concluded that since he was not coughing blood, the bullet had not reached his lung, and so declined suggestions he go to the hospital immediately. Instead, he delivered his scheduled speech with blood seeping into his shirt. He spoke for 90 minutes. His opening comments were, "Ladies and gentlemen, I don't know whether you fully understand that I have just been shot; but it takes more than that to kill a Bull Moose." Afterwards, x-ray showed that the bullet had traversed three inches (76 mm) of tissue and lodged in Roosevelt's chest muscle but did not penetrate the pleura, and it would be more dangerous to attempt to remove the bullet than to leave it in place. Roosevelt carried it with him for the rest of his life

In 1913, he organizes an expedition into the Brazilian jungle to find the headwaters of the Rio da Duvida, the River of Doubt (later it was renamed Roosevelt river). During this trip however suffered a leg wound that gave him tropical fever.

When he returned to New York, he had lost 50 pounds. For the rest of his few remaining years he would be plagued by flare-ups of malaria and leg inflammations so severe that they would require surgery

Toward the end of his life, Roosevelt attempted unsuccessfully to get Wilson to offer him a commission so he could lead a new group of volunteers to fight in World War I.

On January 6, 1919, Roosevelt died in his sleep at Oyster Bay of a coronary thrombosis (heart attack), preceded by a 2½-month illness described as inflammatory rheumatism. The U.S. vice president, Thomas R. Marshall, said that "Death had to take Roosevelt sleeping, for if he had been awake, there would have been a fight."

Theodore Roosevelt is considered the first modern American president. He was also one of the most dynamic, popular, important, and controversial. During his years in office he greatly expanded the power of the presidency.

He is also remembered for his exuberant personality, range of interests and achievements, as well as his "cowboy" persona and robust masculinity.

Roosevelt is credited for changing the nation's political system by permanently placing the presidency at center stage and making character as important as the issues. His notable accomplishments include trust busting and conservationism. For his aggressive attacks on large corporation and trusts over his two terms, he has been called a "trust-buster."

Historians typically rank Roosevelt among the top five presidents

Roosevelt was included with George Washington, Thomas Jefferson and Abraham Lincoln at the Mount Rushmore Memorial, designed in 1927 with the approval of Republican President Calvin Coolidge.