He began his legal career as a law clerk to Judge Pierre N. Leval of the United States District Court for the Southern District of New York, from 1986 to 1987, and then served as a law clerk to Justice John Paul Stevens of the United States Supreme Court, from 1988 to 1989.

[7] "The indictment or criminal prosecution of a sitting President would unconstitutionally undermine the capacity of the executive branch to perform its constitutionally assigned functions.

II, p. 1[8]) cited the Moss memorandum in partial justification of Special Counsel Robert Mueller's decision not to indict President Donald Trump.

[16][17] On March 1, 2020, Moss ruled that President Donald Trump unlawfully installed Ken Cuccinelli as acting Director of the United States Citizenship and Immigration Services (USCIS) and that therefore, certain directives related to removal of undocumented immigrants seeking asylum he implemented “must be set aside.”[18] On December 26, 2020, Moss ruled that the United States Justice Department unlawfully rescheduled the execution of Lisa Montgomery, the only woman on federal death row.

[19] On January 1, 2021, a three judge panel of the United States Court of Appeals for the District of Columbia Circuit vacated Moss's ruling.

[20] On July 19, 2021, Moss sentenced Paul Hodgkins, a Florida man who participated in the 2021 United States Capitol attack to eight months in prison.