After service as a federal circuit judge (1892-1900) and as dean of the Cincinnati law school (1898-1900), he was appointed (1900) head of the commission sent to organize civil government in the Philippines, and he was named first civil governor of the Philippine Islands; he did much to better relations between Filipinos and Americans. In 1904 his friend President Theodore Roosevelt appointed Taft Secretary of War. Taft became a close adviser to the President and was prominent in Latin American affairs, conducting the delicate negotiations attending U.S. intervention in Cuba in 1906.

Roosevelt chose Taft as his successor, and the Republican party named him as presidential candidate in the election of 1908, in which he defeated William Jennings Bryan. He was expected to continue Roosevelt's policies, and to a large extent he did. Trusts were vigorously prosecuted under the Sherman Antitrust Act; the Interstate Commerce Commission was strengthened by the Mann-Elkins Act (1910); and Taft's Latin American policy, known as "dollar diplomacy," was to an extent only an enlargement of Roosevelt's Panama policy and the Roosevelt Corollary to the Monroe Doctrine. The emphasis in all these policies had, however, changed. In Latin America, for instance, the accent was on protection of property and interests of Americans abroad rather than on national interest. Members of the Republican party who favored progressive policies were increasingly restive, and the Insurgents movement grew strong.

The administration made positive achievements in the inauguration of the postal savings bank (1910) and the parcel-post system (1912), and the creation of the Dept. of Labor (1911). Nevertheless, Taft was generally at odds with the progressive elements in his party: he failed to support the Insurgents' attempt to oust the dictatorial speaker of the House of Representatives, Joseph Cannon; he favored the Payne-Aldrich tariff, a high-tariff measure that was denounced by progressive Republicans; and he supported Richard Ballinger against Gifford Pinchot in the Ballinger-Pinchot controversy.

Meanwhile, Taft's relations with Roosevelt deteriorated, and the former President joined the opposition to Taft. In 1912, Roosevelt fought vigorously for the Republican presidential nomination. When he failed and Taft got the nomination, Roosevelt headed the Progressive party and ran in the election as the Progressive (popularly called the Bull Moose) candidate. The Republican vote was split, and the Democratic candidate, Woodrow Wilson, won.

Taft retired from public life and taught law (1912-21) at Yale. He was cochairman (1918-19) of the War Labor Conference in World War I. In 1921, President Harding appointed him chief justice. His chief contribution to the Supreme Court was his administrative efficiency.