Greenberg was nominated by President Ronald Reagan to fill a seat on the United States Court of Appeals for the Third Circuit vacated by Judge Leonard I. Garth on February 11, 1987, and was confirmed by the United States Senate on March 20, 1987. He received his commission on March 23, 1987, and assumed senior status on June 30, 2000.
Notable decisions
''Kreimer v. Morristown''
Richard R. Kreimer, a homeless man residing in various public places throughout Morristown, New Jersey, filed a pro se complaint in the District Court of New Jersey, claiming that his expulsion from the Joint Free Public Library of Morristown and Morris Township was unconstitutional, as it violated his First and Fourteenth Amendment rights. The district court accepted Kreimer’s arguments, holding that the library’s rules and regulations violated the First Amendment as they were unconstitutionally overbroad and unconstitutionally vague. The library’s policies violated the Fourteenth Amendment by making unconstitutional distinctions between patrons. In an opinion delivered by Judge Greenberg, the Third Circuit reversed, finding that libraries are "limited public fora." Given that the library would not have been open without the consent of a majority of voters, the municipality had an interest in assuring the library would be used for its intended purpose; "the communication of the written word." Libraries, by nature, are meant to be places of quiet thoughtful writing, reading or contemplation, and policies designed to protect the intended uses of libraries do not violate the constitution. Any patron who disrupted others and prevented them from using a library to the fullest extent through their actions or inactions could be subject to expulsion.
Dissent in ''LePage's v. 3M''
LePage's sued 3M for anti-trust violations in a case involving 3M's product scotch tape, which 3M had a monopoly over. LePage's asserted that 3M was "bundling" its products together in order to maintain its advantage in the tape market. LePage's won a jury verdict, and 3M appealed to the Third Circuit Court of Appeals. The court heard the case en banc, with Judge Dolores Sloviter writing the opinion affirming the verdict for LePage's. Judge Greenberg dissented from the full Court's opinion. He argued that companies should be able to give rebates to volume purchasers and that the majority was simply punishing 3M for being more efficient than LePage's. He was joined in his dissent by Chief Judge Anthony Joseph Scirica and then-Judge Samuel Alito.
''ACLU v. Mukasey''
In 1998, a federal district court issued a preliminary injunction forbidding the U.S. government from implementing the new Child Online Protection Act. The U.S. Supreme Court eventually upheld this injunction on appeal. This preliminary injunction, however, only lasted until a full trial could be held. In 2007, the full trial was completed, and the district court ruled the law unconstitutional. The government appealed the decision to the Court of Appeals for the Third Circuit. The Third Circuit, in an opinion written by Judge Greenberg, held that the law was not "narrowly tailored" to achieve its purpose of protecting children from pornography. For example, web publishers would have to incur "high costs" in order to avoid prosecution, even when web publishers only served adults. The Supreme Court refused to hear an appeal from Judge Greenberg's ruling, making his decision the last word as to the constitutionality of the law.
