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Mark Harris

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Mark Harris is head of the White Collar Defense & Investigations Group and co-head of the Appellate Group. Mark is also a former federal prosecutor and law clerk at the U.S. Supreme Court. An experienced white-collar criminal defense lawyer, he represents companies and individuals in their most complex and difficult litigation matters.

Mark’s appellate cases span the gamut from intellectual property and labor relations to constitutional law and administrative law. Since 2017, Mark has represented the Financial Oversight and Management Board for Puerto Rico—the entity created by Congress to oversee Puerto Rico’s bankruptcy, the largest in American history—in dozens of appeals before the U.S. Court of Appeals for the First Circuit. In May 2023, he prevailed before the Supreme Court in an 8-1 decision that recognized the Board’s immunity from suit. He is a Fellow of the American Academy of Appellate Lawyers and a past American Lawyer Litigator of the Week.

Mark also maintains an active criminal docket in cases covering every form of financial crime and civil enforcement, including internal investigations. Clients draw on his experience as a former Assistant U.S. Attorney for the Southern District of New York, where he specialized in fraud cases and tried cases before federal juries. Mark is also a recognized expert on criminal sentencing, as a member of the Board of Editors of the Federal Sentencing Reporter, the leading legal journal devoted to the study of sentencing law and policy, for over 25 years.

Mark is the editor and lead author of Principles of Appellate Litigation: A Guide to Modern Practice (PLI Press), a comprehensive treatise on appellate practice, updated every year, which has been described as “invaluable,” “the product of deep experience and keen insights,” and “a superior appellate practice hornbook.”

He has lectured on both criminal law and appellate practice before the International Bar Association, the National Association of Criminal Defense Lawyers, PLI, and the ABA Sections of Litigation, Criminal Law, and Employment and Labor Law. Mark has been interviewed by Bloomberg Radio, the National Law Journal, WINS AM-1010, Law360, Legal Times, and other news organizations.

Mark is a former clerk to U.S. Supreme Court Justices John Paul Stevens and Lewis Powell, Jr., and Judge Joel Flaum of the U.S. Court of Appeals for the Seventh Circuit. He is a graduate of Harvard College and Harvard Law School, where he was a member of the Harvard Law Review. He also serves on the Board of Trustees of the National Museum of Mathematics.

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On August 24, 2020, the United States Court of Appeals for the Fourth Circuit, sitting en banc, reinstated defendant Ronnie Long’s petition for a writ of habeas corpus, challenging his rape conviction more than four decades earlier.  Proskauer filed an amicus curiae brief in support of Mr. Long on behalf of some forty leading scholars who specialize in forensic science, emphasizing the grave impact of the prosecution’s repeated failures to disclose all the forensic evidence in the case.  The Fourth Circuit agreed, and now Mr. Long is expected to be released imminently.

Over forty years ago, Mr. Long was accused of committing a rape and burglary that he has consistently maintained he did not commit.  Relying heavily on the victim’s identification testimony, and the asserted “honesty” of law enforcement who investigated the crime, a jury found Mr. Long guilty of first-degree rape and first-degree burglary.  He was sentenced to life in prison, and his conviction was upheld on appeal.  As the result of continued litigation over the span of many decades, however, a steady stream of suppressed evidence concerning the crime, neither disclosed to the defense nor presented to the jury, came to light. It included lab-test results demonstrating that Mr. Long was not linked to the crime scene; medical evidence taken from the victim that unaccountably went missing; and, most recently, 43 latent fingerprints lifted from the scene, none of which matched Mr. Long.  It also became plain that the detectives who investigated the crime lied at trial about the evidence suppression.