[2] Katzmann served as a law clerk to Judge Hugh H. Bownes of the United States Court of Appeals for the First Circuit from 1980 to 1981.

[3] Katzmann was nominated by President Bill Clinton on March 8, 1999, to a seat on the United States Court of Appeals for the Second Circuit vacated by Judge Jon O. Newman.

[5] In August 2017, Katzmann upheld the insider trading conviction of Mathew Martoma, over the dissent of Judge Pooler, who argued that the majority was improperly overruling circuit precedent.

[8][9] In a landmark Second Circuit ruling, Zarda v Altitude Express (2018) Judge Katzmann, writing for the full court in a 10–3 decision, held that Title VII of the Civil Rights Act of 1964 prohibits employment discrimination on the basis of sexual orientation.

That decision was later affirmed by the Supreme Court of the United States in the consolidated case of Bostock v. Clayton County.

In Corren v. Condos (2018), Judge Katzmann upheld Vermont’s public financing campaign laws against First Amendment challenges.

In the Second Circuit's Trump v. Vance (2019) opinion, Judge Katzmann, writing for a unanimous three judge panel, held that the president is not immune from the enforcement of a state grand jury subpoena directing a third party to produce non-privileged material, even when the subject matter under investigation pertains to the President and that a state grand jury may permissibly issue subpoenas in aid of its investigation of potential crimes committed by persons within its jurisdiction, even if that investigation may in some way implicate the President.

Katzmann wrote articles on a variety of subjects, including judicial-congressional relations, statutory interpretation, the administrative process, regulation, court reform, access to justice for immigrants, civic education, disability, and the war powers resolution.

He served as co-chair of the FTC transition team, and as special counsel to Senator Daniel Patrick Moynihan on the confirmation of Justice Ruth Bader Ginsburg.

[26] The New York Times reported on Katzmann's efforts to foster effective legal representation of the immigrant poor with worthy claims.

Conferees included three Supreme Court Justices – Breyer, Sotomayor and Gorsuch – as well as judges and educators from across the country.

[43] During his term as Chief Judge, the Second Circuit Court of Appeals engaged in a year-long 125th anniversary retrospective on the history of the court, a collaborative effort of judges, staff, and the bar, resulting in a book of judicial biographies,[44] a volume on the jurisprudence of the Second Circuit,[45] and a variety of public programs.

[51] Praised by Justice John Paul Stevens (retired), "as illuminating and convincing" and "required reading for all lawyers confronting questions of statutory construction," the book has been the subject of several commentaries,[52][53][54][55][56][57][58][59][60][61][62] and programs.