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.
Stéphane is recognized by Lexology Index as a Recommended Lawyer in Competition (2025) and is listed in Global Competition Review’s GCR 100 (2025). He is also recommended by The Legal 500 EMEA for Competition for 2023–2024. Clients describe Stéphane as “an excellent competition lawyer with a great strategic sensibility” and praise him as being “great to work with,” noting his “fantastic work product.”
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.
