Robert D. Sack has, since August 6, 1998, been a Judge of the United States Court of Appeals for the Second Circuit with his chambers at 40 Foley Square. He took senior status on August 6, 2009. During his 33 years in practice, he specialized in national and international press law. He was a frequent lecturer and writer on press law topics and is the author of SACK ON DEFAMATION: LIBEL, SLANDER, AND RELATED PROBLEMS (5th ed. 2017) (updated annually) and coauthor with P. Cameron DeVore of the first edition of ADVERTISING AND COMMERCIAL SPEECH: A FIRST AMENDMENT GUIDE (1999). His articles include, Reflections on the Wrong Question: Special Constitutional Privilege for the Institutional Press, 7 Hofstra L. Rev. 629 (1979), Protection of Opinion Under the First Amendment: Reflections on Alfred Hill, "Defamation and Privacy Under the First Amendment," which was published in the 100th Anniversary issue of the Columbia Law Review, 100 Colum. L. Rev. 294 (2000), and New York Times Co. v. Sullivan – 50-Year Afterwords, 66 Ala. L. Rev. 273 (2014). On May 1, 2008, Judge Sack was awarded the Federal Bar Council's Learned Hand Medal for excellence in federal jurisprudence.

Judge Sack was an officer and director of the William F. Kerby and Robert S. Potter Fund, which assisted in funding the legal defense of journalists abroad, and a member of the advisory boards of the Bureau of National Affairs' Media Law Reporter and the ABA Forum Committee's Communications Lawyer. He is a member of the Board of Visitors of the Columbia Law School and was a member of the Board of Trustees of Columbia University Seminars on Media and Society. He has, since 2001, been an Adjunct Professor of Law at Columbia Law School. He was Columbia Law School's commencement speaker in 2007. He was Adjunct Professor of Political Science and Special Guest Lecturer at the University of Rochester in 2012 and a Distinguished Visiting Jurist at the University of Chicago Law School in 2013. He is a member of the American Bar Association, the New York City Bar Association (Chair, Communications Law Committee, 1986-89), and the American Judicature Society. He is a Fellow of the American Bar Foundation.

Judge Sack is an Adviser, American Law Institute's Restatement Third of Torts, Defamation and Privacy, 2019- ; an Adviser to the "Global Media Freedom Initiative's High-Level Panel of Legal Experts" convened at the request of the UK and Canadian governments, 2019- ; and a member of the Editorial Board of the Journal of Free Speech Law, 2021- .

After graduating from the University of Rochester in 1960 and Columbia Law School in 1963, Judge Sack clerked for United States District Judge Arthur S. Lane, in the District of New Jersey. He later became an associate and then partner at Patterson, Belknap, Webb & Tyler (1964-1986) and a partner at Gibson, Dunn & Crutcher (1986- 1998), both in New York City. In 1974, he served as a Senior Associate Special Counsel to the United States House of Representatives Impeachment Inquiry Staff.