William L. Thomas is counsel and head of the firm’s Environment, Health & Safety Practice. Bill advises companies, financiers, developers and other enterprises on environmental law matters, including transactions, projects, and formulation of management and legal compliance strategy. He also handles litigation and disputes involving environmental, health and safety concerns. His expertise in international environmental law and policy matters includes analyzing risks and opportunities arising under regional and international environmental agreements, requirements under emerging EHS legal regimes and related international standards and codes of conduct, and legal aspects of sustainability, corporate social responsibility, and Environmental, Social, Governance (ESG) matters.

Bill is also highly regarded in the area of climate change law, policy and strategy. Specific experience in this field includes: conducting greenhouse gas (GHG) due diligence, applying carbon and other related environmental standards and drafting and negotiating related provisions in transactional and financing documents in connection with mergers, acquisitions, financing of projects and other ventures. He also advises clients on international, federal and state GHG regimes and initiatives and counsels companies, utilities and financiers on emissions trading activities, GHG emission reduction projects, voluntary carbon market issues and related governance matters, including policies, management systems, disclosures, and offsetting activities.

Bill is ranked in Band 1 for Environmental (Transactional) by Chambers USA (2024). He is also consistently ranked among the leading individuals practicing in the area of environmental law by The Best Lawyers in America, The International Who's Who of Environmental Lawyers, The Euromoney Energy and Environment Expert Guide, and Super Lawyers (DC), among others.

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Prior to joining Willkie, Bill was with Skadden, Arps, Slate, Meagher & Flom LLP, and Americas Environment Head for Clifford Chance LLP.

  • Fellow (2010-present) and member of the Board of Regents, American College of Environmental Law (2021-2023)
  • Fellow, American Bar Foundation (2007-present)
  • Chambers USA Environmental: Mainly Transactional (2007-present) and Climate Change Nationwide (2007-2017)
  • Chambers Global Climate Change (2011-2017)
  • Best Lawyers in America Environmental Law and Environmental Litigation (2007-present)
  • Super Lawyers Top Rated Environmental Law Attorney (2013-present)
  • Who’s Who Legal: Thought Leader USA Climate Change and Environmental Law (2023) 
  • Who’s Who Legal: Global Leader Climate Change and Environmental Law (2022)
  • Law Dragon 500 Leading Environmental & Energy Lawyers (2021)
  • Euromoney Energy and Environment Expert Guide (2018)
  • Member of Law360's Environmental Editorial Advisory Board (2016)
  • The Legal 500 United States Environment: Transactional (2021-2023) 
  • The Legal 500 United States Environmental Law (2011- 2012)
  • Practical Law Company Which Lawyer? Environmental Law (2006-2012)
  • Euromoney Guide to the World's Leading Environmental Lawyers (2007)
  • International Who’s Who of Environmental Lawyers (2006-present)

Bill is a member of the American Bar Association's (ABA) Sections on Environment, Energy and Resources (SEER) and International Law (SIL), having previously served on the governing Council for both sections, as well as past Chair of the International Environmental Law committees of both SEER and SIL.  He also previously served as Liaison for SEER and SIL to the ABA Standing Committee on Environmental Law, Vice Chair of the Environmental, Health and Safety Law Committee of the International Bar Association, Vice President of the Environmental Law Protection Law Committee of the Inter-American Bar Association, and Vice Chair of the Environmental Law Committee of the Inter-Pacific Bar Association, and Director of The Conference Board’s Environmental, Health & Safety Legal Council. He is currently a member of the International Environmental Law and Environmental Business Transactions committees of the Environmental Law Section of the New York State Bar Association.      

Bill is a Fellow of both the American College of Environmental Lawyers (ACOEL) and the American Bar Foundation, and completed a three-year term on the ACOEL Board of Regents in 2023. Other appointments include service on the boards of directors of various non-profit organizations, as well as on the Leadership Council of the Environmental Law Institute and the External Advisory Board of the Graham Sustainability Institute, based at the University of Michigan.

Bill also serves as a member of several other bodies, including the editorial advisory boards of several publications, including the Environmental Law Institute's Environmental Law Reporter, Environmental Liability Law, Policy and Practice, and Carbon & Climate Law Review, and is a past member of Law360’s Environmental Editorial Advisory Board.  


Bill appears frequently at national and international conferences, and is regarded as a thought-leader on environmental law, policy and related strategies, including sustainability and ESG matters. In 2007, he served as Chair of ABA SEER's annual Conference on Environmental Law. 


  • “California’s Comprehensive Climate Accountability Regime: Setting an Aggressive New National Standard,” Harvard Law School Forum on Corporate Governance (November 2023) (with William Stellmach and Adam Aderton)
  • “SEC Proposes Climate Disclosure Rules,” The Investment Lawyer, Vol. 29, No. 8 (August 2022) (with Athena Eastwood, Dennis A. Fallon, Elizabeth P. Gray, Robert B. Stebbins, and William J. Stellmach)
  • “From Copenhagen, An Arduous Path Forward,” The Forum (ELI), March/April 2010, at 51
  • “SEC Warms to Climate Change Disclosure,” San Francisco Daily Journal, Feb. 18, 2010, at 7
  • “The SEC Sharpens its Focus,” Environmental Finance, March 2010, at 28
  • “The Road from Bali,” Carbon Business, Spring 2008, at 7
  • “Growing Pains: The Voluntary Offset Market,” Carbon Business, Autumn 2007, at 20
  • “Climate Change at the Capitol,” Environmental Finance, May 2007, at S34
  • “Banking on the Environment: Profiting from Investment in REDD,” 23 Natural Resources & Environment 14 (2009)
  • “The need to be sustainable,” 25 International Financial Law Review 62 (2006)
  • “Survey of Recent Developments in International Environmental Law,” 40 The International Lawyer 197 (2006)
  • “Survey of Recent Developments in International Environmental Law,” 39 The International Lawyer 191 (2005)
  • “Sustainability Develops within US Companies . . . Gradually,” Corporate Environmental Strategy, Vol. 12, No. 4 (April/May 2005)
  • “The Equator Principles and Project Finance: Sustainability in Practice?,” 19 Natural Resources & Environment 20 (2004)
  • “Equator -- Risk and Sustainability,” Project Finance International Yearbook (January 2004)
  • “Issues of Water Scarcity and Rights for Multinational Companies,” 18 Natural Resources & Environment 31 (2003) (with Vail Thorne)
  • “Whither from Here? American Enterprise and the Journey toward Environmentally Sustainable Globalization after the WSSD,” 12 Review of European Community & International Environmental Law 39 (2003)
  • “Ecoterror: Rethinking Environmental Security after September 11,” 16 NATURAL RESOURCES & ENVIRONMENT 157 (2002) (with Michael J. Penders)
  • “Annual Report on International Environmental Law,” in Environment, Energy and Resources 2001: The Year in Review (American Bar Association 2002)
  • “The Green Nexus: Financiers and Sustainable Development,” 13 Georgetown International Environmental Law Review 899 (2001)
  • “Creating a Favorable Climate for Clean Development Mechanism (CDM) Investment North and South of the Rio Grande,” 15 Natural Resources & Environment 172 (2001) (with Daniel Basurto and Gray Taylor)
  • “Annual Report on International Environmental Law,” in Environment, Energy and Resources 2000: The Year in Review 258 (American Bar Association 2001)
  • “Globalization and Its Discontents,” Environmental Finance, March 2001, at 26
  • “Select Bibliography of Articles and Books on International Environmental Law 1997 to Present,” 13 Georgetown International Environmental Law Review 47 (2000)
  • “Using Auditing, Pollution Prevention, and Management Systems to Craft Superior Environmental Enforcement Solutions,” 30 Environmental Law Reporter 10299 (May 2000)
  • “The Kyoto Protocol: A Factor in Foreign Investment in Mexico?,” Public Utilities Fortnightly, April 15, 1999, at 40
  • “Calculating the Cost of Greenhouse Gas Emission Reductions Heading Into Kyoto Protocol Negotiations In Buenos Aires,” Environmental Quality Management, Autumn 1998, at 29
  • “The Kyoto Protocol and U.S. Climate Change Policy: Implications for American Industry,” 7 Review of European Community & International Environmental Law 189 (1998)
  • “The ASARCO-EPA Settlement Agreement: Understanding the Value of a Corporate-Wide EMS for Regulators and Strategists Alike,” Corporate Environmental Strategy, Summer 1998, at 4
  • “Achieving and Maximizing ISM Code Compliance with ISO 14001,” Environmental Quality Management, Summer 1998, at 9
  • “ASARCO Settlement Attests to Importance of a Sound Environmental Management System,” Natural Resources & Environment, Spring 1998, at 290
  • “Looking for a Way to Reduce Wetland Permitting Delays and Costs? Put Your Money in a Bank,” Metropolitan Corporate Counsel, April 1998, at 8
  • “Devising a Compliance Strategy under the ISO 14000 International Environmental Management Standards,” 15 Pace Environmental Law Review 85 (1997)
  • “Using ISO 14001 to Comply with the Management System Requirements of the U.S. EPA’s RMP Rule and the EU’s Seveso II Directive,” European Environmental Law Review, December 1997, at 337
  • “ISO 14001 Sets a Global Environmental Standard,” National Law Journal, July 8, 1996, at C2 (with Donald A. Carr)
  • “The Law and Policy of Endangered Species Act Reauthorization: Noah’s Choices and Ecological Mandarins,” 25 Environmental Law 1281 (1995) (with Donald A. Carr)


  • “United States” chapter, The Legal 500: Renewable Energy Country Comparative Guide (2022) (with Norman C. Bay, Jorge Kamine, Matthew Vittorla, and Juliana V. Pimentel) 
  • Crafting Superior Environmental Enforcement Solutions (ELI 2000) (with Bertram Frey and Fern Fleischer Daves)
  • “United States” chapter, Cross-Border Environment Handbook (PLC 2006-2012 editions)
  • “Rio’s Unfinished Business: American Enterprise and the Journey toward Environmentally Sustainable Globalization,” in Stumbling Toward Sustainability (John Dernbach ed., ELI 2002)
  • “Introduction” and “United States” chapters, in Environmental Law and Enforcement in the Asia-Pacific Rim (Terri Mottershead ed., Sweet & Maxwell 2002)

William Thomas advised on necessary environmental aspects on the following matters: 

  • Odinsa S.A., a major developer of transportation projects in South America, in the sale of a 50% stake in certain of its airport concessions in Colombia and Ecuador to a Macquarie Asset Management (MAM) affiliate as part of the expansion of the infrastructure platform the two partners formed to operate and develop their South American road projects.
  • Carrix in the acquisition of Ceres Terminals.
  • Atlas Holdings in its sale of a majority stake in International Wire Group Holdings to Olympus Partners.
  • Platinum Equity in connection with various private equity matters, including the acquisitions of Tarter Farm and Ranch Equipment, MetoKote Corporation, San Diego Union-Tribune and Aquilex.
  • Blue Wolf Capital Partners in its acquisition of Sterling Site Access Solutions.
  • Essent Group, Ltd., a mortgage guaranty insurance and reinsurance provider, in its acquisitions of Agents National Title Holding Company and Boston National Holdings LLC.
  • Fresh Express Incorporated, a wholly owned subsidiary of Chiquita Holdings Limited, in the acquisition of Dole plc’s Fresh Vegetables Division.
  • Hub Promotional Group (HPG), a portfolio company of Tenex Capital Management, in its acquisition of Evans Manufacturing.
  • iCON Infrastructure LLP in its strategic partnership with Royal Caribbean Group.
  • Second Nature Brands, a portfolio company of CapVest Partners, in its acquisition of Brownie Brittle.
  • Avista Capital Partners in its acquisition of Spear Education, a provider of clinical training, practice analytics and consulting solutions for the dental market.
  • Choice Hotels International in its approximately $675 million acquisition of Radisson Hotel Group Americas.
  • Frontline Road Safety, a portfolio company of The Sterling Group, in its acquisition of the pavement marking contracting operations of Ozark Striping Company, LLC.
  • The Bonduelle Group in its sale of a 65% stake in Bonduelle Americas Long Life to Fonds de solidarité FTQ and Caisse de dépôt et placement du Québec
  • Blackstone Credit in connection with financing for Permira’s approximately $5.8 billion take-private of Mimecast Limited.
  • DigitalBridge Group in the $3.2 billion sale of its Wellness Infrastructure business to Highgate Capital Investments and Aurora Health Network.
  • FFL Partners in its investment in Perlman Clinic, a leading independent provider of primary care in the Greater San Diego market.
  • Lakeside Book Company, an Atlas Holdings platform company, in its acquisition of Phoenix Color Corp.
  • Resideo Technologies in its $593 million acquisition of First Alert.
  • Curtis Instruments in its sale to Kohler Co.
  • Tenex Capital Management in its sale of AquaCentral, a leading distributor of pool equipment and supplies, to Heritage Pool Supply Group, a platform within SRS Distribution, a portfolio company of Leonard Green & Partners, L.P. and Berkshire Partners.
  • CMA CGM Group in its acquisition of most of Ingram Micro’s Commerce & Lifecycle Services business.
  • The Sterling Group in its sale of Time Manufacturing to H.I.G. Capital.
  • Franchise Group in its acquisition of W.S. Badcock Corporation.
  • Atlas Holdings in its planned acquisition of R.R. Donnelley
  • A Brookfield-sponsored private real estate fund in the strategic partnership with King Street Properties to pursue investment in life science real estate across the US, including an investment of approximately $1.5 billion in equity capital towards King Street's life science real estate pipeline.
  • PAI Partners in its $3.3 billion pending acquisition of Tropicana, Naked and other select juice brands across North America from PepsiCo, Inc.
  • FFL Partners and Two Sigma Impact in their acquisition of a majority stake in Community Medical Services.
  • Orion Advisor Solutions in its acquisition of Redtail Technology, a leading web-based CRM software firm serving the wealth management industry. 
  • Famille C, the Courtin-Clarins family holding company and owner of the Clarins brand, in its acquisition of ILIA Beauty.
  • WebPT, a portfolio company of Warburg Pincus, in its acquisition of Clinicient.
  • A syndicate of lenders in a $123.5 million term loan acquisition financing to a borrower formed to be an owner and operator of fully integrated environmentally clean bitcoin mining facilities in the United States.
  • Tenet Healthcare Corporation and its subsidiary United Surgical Partners International in the planned acquisition of SurgCenter Development’s SCD’s ownership interests in 92 ambulatory surgery centers (ASCs) for $1.2 billion.
  • SG Energy, a US energy company and a fully owned subsidiary of Supergas Energy, an Israeli energy company traded on the Tel Aviv Stock Exchange, in its investment in Balanced Rock Power, a Utah-based company that develops, invests in, and sells renewable energy projects throughout North America.
  • New Mountain Capital and DRB Systems in DRB’s sale to Vontier for approximately $965 million.



Georgetown University Law Center, J.D., 1989 St. Olaf College, B.A., 1986

Bar Admissions

District of Columbia New York