Katō's interest in Western studies developed at a time when Japan was still isolated from the outside world—in the late 1850s and early 1860s, when Katō worked in the government office for the study of foreign books. In 1864, when that office was made into a school for the study of Western learning, Katō became a professor of foreign affairs. After the 1868 Meiji Restoration abolished the old Japanese feudal regime of the shogunate, he became a private tutor to the emperor and was appointed to many high government posts in education and foreign affairs. Meanwhile, through such books as Shinsei taii (1870; “General Theory of True Government Policy”) and Kokutai shinron (1874; “New Theory of the National Structure”), he introduced the Japanese public to European theories of government, democracy, and human rights.

About 1880, however, when the movement for parliamentary democracy began to gain momentum, Katō altered his earlier views, arguing that it was too early for a national assembly and that, as Prussia demonstrated, democracy was not necessary for national strength. The constitution that was finally promulgated in 1889 was based on the Prussian, not the British or French, model. The Meiji constitution also held that human rights were not inalienable but a privilege granted by the state, a position taken by Katō in his Jinken shinsetsu (1882; “New Theory on Human Rights”).

With the founding of Tokyo Imperial University in 1890, Katō became its first president. In 1900 he was created a baron and in 1906 was made a member of the Privy Council, a position that heightened his influence on state policy. By that time he had gained international recognition from the 1893 German publication of his Der Kampf ums Recht des Stärkeren und seine Entwickelung (1893; “War, Right of the Strongest, and Evolution”).