Along with Max Brauer, a Social Democrat and mayor of Altona, Katz became an envoy for municipal administration of the League of Nations in Nanjing in October 1933.

[1][4][5] In 1935 he moved to the United States and worked at the Columbia University's Institute for Public Administration and as a journalist for the Neue Volkszeitung, a German-language newspaper of Social Democrat emigrants.

Katz was a director of the Rand School of Social Science in New York and of The New Leader newspaper, he was active in the German Labour Delegation, part of the American Federation of Labor, and the "German-American Council for the Liberation of Germany from Nazism".

[2][4] After Katz had lost his German citizenship because of the racialist Nazi laws, he became a United States citizen in 1941.

[2] In July 1946 Katz returned to Germany along with Max Brauer and became Minister of Justice (1947–50) and Education (1948–49) in the State of Schleswig-Holstein.