O’Melveny Worldwide

Rebecca Mermelstein is a first-chair trial lawyer with extensive experience handling white collar and finance-related investigations and litigation. She advises corporate and individual clients on complex white collar matters, including regulatory actions, internal investigations, and compliance initiatives.

Prior to joining O’Melveny, Rebecca spent more than a decade as an Assistant U.S. Attorney in the U.S. Attorney’s Office for the Southern District of New York. She was a senior member of the Securities and Commodities Task Force before rising to Co-Chief of the General Crimes Unit. Rebecca has tried 12 cases to verdict, briefed and argued numerous federal appeals, and conducted or supervised more than 1,000 investigations, including grand jury investigations and cross-border matters involving simultaneous investigations by multiple U.S. and international regulators. Her expertise spans securities and accounting fraud, insider trading, Ponzi schemes, money laundering, cybercrime, public corruption, human trafficking, and sexual misconduct.

Through her work leading investigations and prosecutions, Rebecca has established strong working relationships with U.S. Attorney’s Office colleagues and officials at the Securities and Exchange Commission, Commodity Futures Trading Commission, and the Public Company Accounting Oversight Board, as well as federal law enforcement agencies. The knowledge she has gained from her experience and relationships helps her achieve the best possible results for clients.

  • Represent a global commodities trading company in a wide-ranging FCPA investigation by the United States Attorney’s Office for the Eastern District of New York and Main Justice involving alleged conduct in South America.
  • Represent a Fortune 50 company in a DOJ investigation under the False Claims Act.
  • Represented an entertainment company in an internal investigation of senior officers concerning possible violations of internal finance and approval policies.
  • Represented a global consulting firm in an in internal investigation and litigation relating to allegations under the Adult Survivors Act.
  • Secured an acquittal for a Louisiana man who served more than eight years in prison after he was wrongfully convicted of second-degree murder by a non-unanimous jury.

Government Experience

  • Supervised dozens of jury trials and more than 1,000 cases involving securities fraud, wire fraud, mail fraud, tax fraud, money laundering, access device fraud, cybercrime, immigration offenses, child exploitation, human trafficking, robbery, narcotics trafficking, and firearms offenses.
  • Investigated and prosecuted complex financial crimes, including accounting fraud, insider trading, market manipulation, investment adviser fraud, wire fraud, bank fraud, insurance fraud, and money laundering. Managed relationships with FBI and SEC, among other agencies.
  • Conducted 12 criminal trials, including numerous multi-defendant white-collar cases. Briefed and/or argued more than a dozen appeals in the Second Circuit.
  • Investigated, prosecuted, and supervised numerous sex-crimes cases, including leading the investigation and prosecution of a 19-defendant international sex-trafficking ring and supervising multiple cases of sex abuse by middle school and high school teachers. 


Bar Admissions

  • New York
  • New Jersey

Court Admissions

  • United States District Court, Southern District of New York
  • Second Circuit Court of Appeals


  • Harvard Law School J.D., 2005: cum laude
  • Columbia College, B.A., Comparative Religion, 2001: magna cum laude; Phi Beta Kappa 

Honors & Awards

  • Recommended by The Legal 500 US for Corporate Investigations and White-Collar Criminal Defense (2024)

Professional Activities


  • Recurring guest on the podcast Against the Rules with Michael Lewis: The Trial of Sam Bankman-Fried
  • Guest on NPR’s news program All Things Considered
  • Featured, “Litigator of the Week: After SCOTUS Reverses Non-Unanimous Murder Conviction, O’Melveny Wins Acquittal on Retrial,” The American Lawyer (March 17, 2023)
  • Co-Author, “Turning the Tables on DOJ: Individual Defendants Wield Corporate Criminal Enforcement Policies Against the Government,” New York Law Journal (March 6, 2023)


  • Honorable Charles P. Sifton, Eastern District of New York