Stéphane Dionnet - McDermott Will & Emery


Stéphane Dionnet is co-head of McDermott’s Antitrust Mergers Focus Group and has over 20 years of experience practicing international competition law, and has experience liaising with competition authorities in the European Union, United States and Asia. Stéphane has advised global corporations on how to navigate complex multijurisdictional merger control matters. He has represented clients in numerous cartel investigations and has also successfully assisted companies in obtaining conditional immunity from the European Commission and other competition law agencies in and outside the European Union.

Stéphane has experience across a wide range of sectors including consumer products, energy, entertainment, financial services, technology, pharmaceuticals, retail, e-commerce and telecommunications.

Earlier in his career, Stéphane worked for three years within the “Merger Network” of the European Commission’s Directorate-General for Competition. He was case-handler in high-profile transactions, including phases 1 and 2 with remedies.


  • Advised a provider of voiceover IP products on its $250 million acquisition of a cloud communications software platform*
  • Advised a multinational corporation on its $7.8 billion sale of an 80.1% stake in one of its divisions to another multinational conglomerate*
  • Advised a multinational medical technology company on its $12.2 billion acquisition of a medical corporation*
  • Advised a transport and logistics company on its $4.2 billion acquisition of a global integrated logistics business*
  • Advised an investment management firm on the $12.5 billion business combination of a number of its divisions*
  • Advised a pulp and paper company on its acquisition of two packaging businesses in Europe*
  • Advised a chemicals company on its $2.5 billion acquisition of a global corporation*
  • Advised a global technology investor on its acquisition of European companies*

*Matter handled prior to joining McDermott


  • Global Competition Review, GCR 100, 2024
  • The Legal 500 EMEA, Recommended, 2023-2024
  • Who’s Who Legal Competition 2023, Leading Practitioner
  • Who’s Who Legal Thought Leaders – Global Elite 2023 (Future Leader)
  • Who’s Who Legal Competition Future Leaders, 2022


Stéphane is a member of the International Bar Association Antitrust Committee serving as a member of its Mergers Working Group.


College of Europe Bruges, LL.M., (European Law), 2001
University of Helsinki, LL.M., (Contract and Commercial Law), 2000
University of Paris V, Descartes, D.E.S.S., (European Business Law), 1999
University of Paris I, Sorbonne, Maitrise, (European Law), 1998