• University of Pennsylvania Law School (JD)
  • Howard University (BBA, Finance, summa cum laude)

Bar Admissions

  • District of Columbia
  • New Jersey
  • United States Court of Appeals for the Fifth Circuit
  • United States Court of Appeals for the Sixth Circuit

Dionne’s antitrust and trade regulation practice includes counseling and representation in connection with merger and non-merger related activities, litigation, and matters before the US Department of Justice (DOJ) and the Federal Trade Commission (FTC). Dionne has been involved in antitrust matters spanning a broad range of industries, including the health care and energy sectors.

In the health care industry, Dionne has represented large health systems, major medical associations, large multispecialty physician groups, pharmaceutical companies, and medical device manufacturers in connection with mergers and acquisitions, joint ventures, and other commercial arrangements. She has counseled various clients in responding to antitrust investigations conducted by the DOJ, FTC, and various state antitrust agencies. Dionne is also experienced in international mergers and has assisted clients with premerger notification filings around the world.

Prior to private practice, Dionne served as a Trial Attorney at the DOJ Antitrust Division Health Care Task Force Section. In this position, she handled a number of civil merger and non-merger investigations. Her practice involved reviewing Hart-Scott-Rodino filings, conducting witness interviews and depositions, drafting Second Requests and subpoenas, developing case strategies, and preparing cases for trial.

Following law school, Dionne was a law clerk for the Honorable Clifford Scott Green in the US District Court for the Eastern District of Pennsylvania.

Representative Matters

  • Represented a major master limited partnership (MLP) in the oil and gas industry before the FTC in connection with its successful $1.3 billion sale of midstream assets to a large midstream MLP. 
  • Represented a private equity company before the FTC in connection with its investment in taking one of the largest midstream companies in the US private. Developed firewalls and other compliance measures to comply with the consent decree negotiated with the FTC to resolve competitive concerns.
  • Represented an energy company in an FTC investigation of its $1.1 billion acquisition of a financial interest in another energy company. 
  • Represented a national pharmaceutical distributor before the FTC in its successful $13 billion acquisition of pharmaceutical distribution assets.
  • Represented a national health system before the Pennsylvania State Attorney General resulting in the successful $100 million acquisition of hospital assets in southeastern Pennsylvania. 
  • Successfully defended an international luggage shipping company against antitrust claims in the US District Court for the Eastern District of Texas.
  • Provided antitrust advice to a leading global pharmaceutical manufacturer regarding a joint venture with another pharmaceutical company.
  • Provided antitrust advice to one of the country's largest operators of medical groups and physician networks regarding a joint venture with a large regional health system.
  • Advised an international pharmaceutical client on the antitrust implications of certain patent litigation settlements.
  • Represented an administrator of Medicaid dental programs before the FTC in its successful $95 million transaction with a national commercial dental insurer. 
  • Represented one of the largest multispecialty physician groups in Northern Virginia regarding the structure and operation of various joint ventures and related antitrust issues. 
  • Provided antitrust advice to a national pharmaceutical company and medical device manufacturer in connection with a competitive collaboration
  • Advised a private equity company in connection with the FTC’s investigation of its sale of certain retail assets to a major food producer in the US.
  • Represented a national book publisher before the DOJ in its successful $3.5 billion acquisition of educational publishing assets. 
  • Represented a global producer of titanium dioxide pigments before the FTC in connection with its acquisition of another titanium dioxide producer in a transaction valued at approximately $1 billion. 
  • Represented a global specialty chemicals company before the FTC in connection with its proposed transaction with another global specialty chemicals provider.
  • Provided antitrust advice to a multinational chemical company regarding the acquisition of certain pipeline assets.
  • Represented a software company in the multi-jurisdictional merger review of its $219 million transaction with another software and equipment supplier. 
  • Represented an international manufacturer of liquid analytical instruments before the DOJ in its merger with a national manufacturer of water analytical instrumentation. 
  • Provided antitrust counseling and advice to a global private investment firm in connection with a variety of transactions.
  • Counseled numerous clients on gun jumping, information exchange, and other premerger coordination issues.

Case Citations

  • United States v. Microsoft
  • United States v. Dentsply International Inc.
  • United States v. Long Island Jewish Medical Center and North Shore Health System
  • United States v. Aetna, Inc. and The Prudential Insurance Company of America

Recognitions & Awards

  • City Year Idealist of the Year (2016)
  • Leading Practitioner, Expert Guides  – Competition and Antitrust (2016 – 2018)
  • Washington DC Super Lawyers: (2013 – 2018)
  • Legal 500 United States: M&A Antitrust (2012 – 2013)
  • Chambers USA: District of Columbia – Antitrust (2009 – 2012)
  • Nightingale's Healthcare News: Outstanding Healthcare Antitrust Lawyers (2006)

Professional & Community Involvement

  • Co-chair, Health Law Forum, The Women’s Bar Association of the District of Columbia (WBA) (2014 – 2015)
  • Member, American Bar Association, Antitrust Section, Health Care Committee
  • Board of Trustees, The Legal Aid Society of the District of Columbia (2007 – 2017)
  • American Heath Lawyers Association, Antitrust Practice Group, Vice Chair of Publications