He previously served as the Principal Deputy Assistant Attorney General for the Office of Legal Counsel of the United States Department of Justice.
Gannon clerked for Supreme Court Justice Antonin Scalia during the 2004 Term, and Fifth Circuit Judge Edith Jones.
[2][5] In 2014, Gannon argued the work shift case of Integrity Staffing Solutions, Inc. v. Busk on behalf of the Government.
During oral arguments, Justice Kagan posed a hypothetical to him: There was a federal judge in New York "ages ago .
[7] On January 27, 2017, Gannon issued a memorandum approving the legality of Executive Order 13769, the controversial "refugee ban" limiting immigration from seven countries.