After earning his law degree, Margold returned to New York City in 1923 and set up a private practice. With
John Collier, Margold wrote the solicitor's opinion, "Powers of Indian Tribes" which was issued October 25, 1934, and commented on the wording of the
Indian Reorganization Act. According to
Vine Deloria Jr. and
Clifford M. Lytle, "Modern tribal sovereignty thus [began] with this opinion" because the opinion recognized that the sovereignty of Indian tribes was inherent, rather than being granted to them by the federal government. In 1940, Margold wrote the introduction to the
Handbook of Federal Indian Law by
Felix S. Cohen. Margold believed that Indian self-governance was "a revealing record in the development of our American constitutional democracy." Recognizing his loyalty and legal expertise, Franklin Delano Roosevelt appointed Margold as a judge on the Municipal Court for the District of Columbia in 1942 where he continued to serve until 1945. He was then moved to the U.S. District Court in the District of Columbia and served there until his death in 1947. ==Death==