Dennis Mahony

Dennis E. Mahony

Toronto: 416.865.8214
Fax 416.865.7380


LLB, 1993
University of Toronto
BA, 1990
University of Calgary

Bar Admissions

Ontario, 1995


Dennis is the head of Torys’ Environmental, Health and Safety Practice, the Co-Chair of the firm’s interdisciplinary Climate Change and Emissions Trading Practice and one of the core members of our Infrastructure and Energy Group.

He is certified by the Law Society of Upper Canada as a Specialist in Environmental Law and has been recognized over the years as one of Canada’s leading practitioners, including in: Lexpert/ALM’s Canadian Legal Lexpert Directory; Practical Law Company’s Which Lawyer? Yearbook; Best Lawyers’ Best Lawyers in Canada; Law Business Research’s International Who’s Who of Environment Lawyers; Legal Media Group’s Expert Guides: Best of the Best; Legal Media Group’s Guide to the World’s Leading Environment Lawyers and Chambers & Partners (Band 1). Dennis is also a past recipient of the Lexpert award for Canada’s Top 40 Lawyers Under 40. He is a past Co-Chair of the International Outreach Task Force of the American Bar Association’s Section of Environment, Energy and Resources (SEER), the current International Vice-Chair of the Energy and Environmental Markets and Finance Committee of SEER, and a Past Chair of the Canadian Bar Association’s National Environmental, Energy and Resources Law Section.

Dennis’s environmental, health and safety practice involves work with a broad range of public and private sector clients, including public and private companies, federal government agencies, Crown corporations, provincial bodies and municipalities. He provides advice to clients with respect to virtually every industrial sector, including transportation; utilities; non-renewable and renewable energy; pulp and paper; mining; chemicals and plastics; water and waste water treatment and disposal; manufacturing (including automotive, auto parts, adhesives, sports equipment, injection molding and food); financial institutions (including all of North America’s major banks); and real estate development (including major property and property management corporations).

Dennis’s infrastructure practice involves working with developers, lenders, technical consultants, regulators and other project stakeholders to plan, develop, permit and defend complex projects, often through sensitive environmental habitats. He typically advises from the project planning stage through the approval and construction stages and into operation. His advice is full-spectrum, from acquisition diligence and agreement negotiation, through compliance advice, major and minor environmental assessment and other approvals, strategic stakeholder management, and litigation before environmental tribunals and courts. Over the past several years, Dennis and team have successfully advised and defended more major projects than any other environmental team in the country.

As part of his transactions practice, Dennis assists buyers, sellers, lenders and investors through all the environmental aspects of a transaction, from the design and coordination of deal-specific (often multiple-site, international) environmental liability assessments; through the negotiation of agreements; and in handling closing and post-closing details. In a typical year, the group is involved to varying degrees in more than 100 commercial transactions.

Dennis’s regulatory practice involves technical and strategic advice that spans the full range of issues in the environmental, health and safety field, including project development, particularly with respect to federal and provincial environmental assessments and other approvals; contaminated sites; quasi-criminal, civil and administrative liability assessments (including those related to government orders); transportation of dangerous goods and waste (domestically and internationally); climate change; general legal compliance; legislative initiatives; and Canada-U.S. cross-border issues.

Dennis’s litigation related practice covers an equally broad range of issues, including environmental assessment and approval hearings, appeals of administrative orders, quasi-criminal defence work and civil litigation.

Dennis’s climate change and emissions trading practice involves helping our clients navigate through this evolving, multi-faceted field, including keeping them informed on the numerous domestic and international developments in the area and working on project-specific teams for everything from assessing the ever evolving securities disclosure requirements to emissions trading.

Representative Work

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Chambers Canada—Leading lawyer in environment (2016-2021)

Chambers Global—Leading lawyer in environment (2014-2021)

The Legal 500 Canada—Leading lawyer in environment (2015-2021)

Best Lawyers in Canada—Leading lawyer in environmental law (2006-2021)

The Canadian Legal Lexpert Directory—Leading lawyer in environment (2014-2020)

Who’s Who Legal—Leading Canadian lawyer in environment and climate change (2014-2020)

Euromoney’s Guide to the World’s Leading Energy and Environment Lawyers—Leading lawyer (2017-2018)



Professional Involvement

Dennis is the former co-editor in chief of University of Toronto Faculty of Law Review. He is also an adjunct professor at the University of Toronto Faculty of Law, where he teaches Climate Change law. He regularly chairs and speaks at environmental, health and safety law conferences in Canada and the United States, and has authored and co-authored numerous articles.

Memberships and Affiliations

Dennis is the Co-Chair of the International Outreach Task Force, as well as a member of the Section of Environment, Energy and Resources (SEER) for the American Bar Association. He was also the International Vice-Chair (2009/10) for SEER and the Energy and Environmental Markets and Finance Committee for the American Bar Association, as well as the Immediate Past Chair (2009/10) for the Canadian Bar Association. Dennis is also the Executive for the Environmental Section for the Ontario Bar Association, in addition to being a member of the International Bar Association and the American Society of International Law.