Julian William Mack (1866–1943) was a United States federal judge and social reformer. Born in San Francisco, California, he was raised in Cincinnati, Ohio, attending the public schools there from 1873 to 1884. He received a LL.B. from Harvard Law School in 1887, and was awarded a Parker Fellowship by Harvard University, attending the Universities of Berlin and Leipzig, in Germany, from 1887 to 1890.
Mack was admitted to the Illinois bar in 1890 and was in private practice in Chicago from 1890 to 1895. In 1895, he secured an appointment as a professor of law at Northwestern University. He transferred to the University of Chicago in 1902 and there remained until his retirement in 1940. During his time in Chicago Mack became a member of the city's leading Jewish Reform congregation, Chicago Sinai congregation. Encouraged by its rabbi, Emil G. Hirsch, Mack became the leading manager of the United Hebrew Charities of Chicago during the 1890s.
Mack first served as circuit court judge in Chicago, then as a judge on the Illinois District Appeals Court. In 1910, he was appointed by President William Taft to the US Court of Appeals for the Seventh Circuit and the United States Commerce Court. In 1929, he was reassigned as an additional judge to the United States Court of Appeals for the Sixth Circuit and the United States Court of Appeals for the Second Circuit. The following year, he was reassigned to serve solely on the Sixth Circuit. Mack assumed senior status in 1940, serving in that capacity until his death.
During World War I, he served on the Commission of Labor of the Council of National Defense, the National War Labor Board, and the War Department's Board of Inquiry on Conscientious Objectors. He also coordinated Jewish war relief activities.
Julian Mack was one of the founders of the American Jewish Committee in 1906. His Jewish charitable work included serving as president of the Palestine Endowment Funds, president of the American Jewish Congress, Zionist Organization of America, and various other organizations. He attended the Versailles Conference as an advocate for a Zionist state in Palestine. Kibbutz Ramat ha-Shofet, founded in Israel in 1941, was named in his honor.
At the first Plenary Assembly in Geneva in 1936, Mack was elected as first honorary president of the World Jewish Congress, a position he served in until his death in September 1943.