He was strongly influenced by the Christian socialism of Abe Isoo, and in the 1920s served as a legal adviser to labor organizations and socialist political parties.

[1] After World War II, Katayama began to serve as secretary-general of the Japan Socialist Party when it was established in November 1945.

[2] Despite leading a short-lived administration, Katayama, during his time in office, saw the enactment of a wide range of progressive social reforms such as the establishment of Japan's first Labour Ministry;[3] an Unemployment Compensation Act; an Unemployment Insurance Act; and the overhaul revision of the Civil Code, whose section on the family institution was completely rewritten to provide, for instance, the eldest son with a greater inheritance share.

[5] The Law for the Elimination of Excessive Economic Concentration (passed in December 1947) provided for the dissolution of any company considered to be monopolistic,[9] while the "law on the expulsion of Zaibatsu-affiliated controls" of January 1948 enforced the resignation of Zaibatsu board members who were related closely to Zaibatsu families, while a measure was taken to ban on holding the concurrent board posts of their affiliated companies.

In addition, a government employees law was enacted, the first group of Japanese Supreme Court justices was appointed, local government and the police were reorganised, the Ministries of Home Affairs, Navy, and War were abolished,[10] extensive revisions were made to criminal law, and progress was made on land reform.

[13] After his resignation, Katayama became a member of the Democratic Socialist Party and advocated the maintenance of the pacifist constitution, election reform, and formation of a global commonwealth.

[14][15] As a result, for the first time in human history, a World Constituent Assembly convened to draft and adopt a Constitution for the Federation of Earth.