Dennis Mahony > Torys > Toronto, Canada > Lawyer Profile


Work Department



A leading lawyer in his field, Dennis has specialized knowledge in environmental law. From regulatory compliance to courtroom advocacy, Dennis works closely with clients on bespoke tactics to mitigate risk and maximize opportunity in environmental matters impacting clients’ operations, projects and transactions.

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 certified by the Law Society of Ontario as a Specialist in Environmental Law.

Dennis provides comprehensive environmental, health and safety advice to a broad range of public- and private-sector clients, including public and private companies, federal government agencies, Crown corporations, provincial bodies and municipalities. The scope of his counsel covers virtually every industrial sector: transportation, utilities, non-renewable and renewable energy, pulp and paper, mining, chemicals and plastics, water and waste water treatment and disposal, manufacturing, financial institutions and real estate development.

On projects, Dennis works alongside developers, lenders, technical consultants, regulators and other project stakeholders to plan, develop, permit and defend complex projects, often through sensitive environmental habitats. He regularly advises from the project planning stage through the approval and construction stages and into operation. His broad advice to clients spans regulatory compliance, 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 various Torys teams have successfully advised and defended more major projects than any other environmental team in the country.

Dennis also assists buyers, sellers, lenders and investors through all environmental aspects of a transaction: from acquisition diligence and agreement negotiation, to the design and coordination of deal-specific environmental liability assessments, negotiation of agreements and in handling closing and post-closing details.

Dennis has a broad regulatory practice that involves technical and strategic advice spanning the full range of issues in the environmental, health and safety field. In particular, he advises on federal and provincial environmental assessments and other approvals, contaminated sites, quasi-criminal, civil and administrative liability assessments, transportation of dangerous goods and waste, climate change, general legal compliance, legislative initiatives and Canada-U.S. cross-border issues. His work regularly involves litigation, covering environmental assessment and approval hearings, appeals of administrative orders, quasi criminal defence work and civil litigation.

With respect to climate change and emissions trading, Dennis helps clients navigate through this evolving area, including keeping them informed on the numerous domestic and international developments in the area and working on project-specific teams for everything from assessing securities disclosure requirements to emissions trading.


Dennis is the former co-editor in chief of University of Toronto Faculty of Law Review and is 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 U.S., and has authored and co-authored numerous articles.

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 for SEER and the Energy and Environmental Markets and Finance Committee for the American Bar Association, as well as the Immediate Past Chair for the Canadian Bar Association. He 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.


Ontario Bar, 1995

Lawyer Rankings

Canada > Environment

(Leading individuals)

Dennis MahonyTorys