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) and coauthor of ADVERTISING AND COMMERCIAL SPEECH: A FIRST AMENDMENT GUIDE (1999). His article, Protection of Opinion Under the First Amendment: Reflections on Alfred Hill, Defamation and Privacy Under the First Amendment," was published in the 100th Anniversary issue of the Columbia Law Review. 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 assists 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, and was Chairman of the National Council on Crime and Delinquency. He has since 2001 been a Lecturer in 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.

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 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.