Dan is a trial lawyer who represents states, cities, and communities in high-stakes environmental and consumer protection cases. A member of Keller Rohrbacks Complex Litigation Group and a leader in the firms environmental practice, Dan builds teams confronting major contamination, natural resource damages, and other public harms.
Dan has played a central role in litigation against PacifiCorp arising from its 2020 Labor Day fires in Oregon. Dan helped initiate the case and led the team in one of the ensuing damages trials, securing a judgment of more than $70 million for 12 fire victims. The corporation now faces judgments exceeding $1 billion.
Dan also represented the State of Oregon in its PCB litigation against Monsanto, which resulted, on the eve of trial, in a $698 million settlement. After several years of litigation, Dan helped the City of Seattle recover $160 million against Monsanto for PCB litigation in the Duwamish River. He currently represents several other states and counties in PCB litigation.
Across these matters and others, through public nuisance and class actions, Dans work has focused on holding corporations accountable for widespread harm. Within that work, Dan is recognized for building cohesive teams, encouraging collaboration, and seeing cases through to trial.
Before joining Keller Rohrback, Dan taught environmental law at Lewis & Clark Law School. His academic work focused on toxic waste, water pollution, and natural resource management, and he has contributed to environmental rulemaking and governmental advisory efforts.
Articles & Presentations
•Panelist, Government as Plaintiff: The Advantages and Hurdles of Litigating on Behalf of a Public Body,Coalition of Civil Advocates (November 14, 2024).
•Speaker, PFAS from the Litigation Perspective, Law Seminars International, A Comprehensive Seminar on the 33rd Anniversary of the Model Toxics Control Act (December 7, 2023).
•Panelist, Toxic Torts and Class Actions, Environmental Law and Policy Conference, University of Michigan, Michigan Law (September 30, 2022).
•Panelist, Accountability for Climate Change Harms in the Pacific Northwest: Scientific, Policy and Legal Perspectives, Lewis & Clark Law Schools Green Energy Institute, the Center for Climate Integrity, and Breach Collective (March 18, 2021).
•Speaker, Alliance of California Judges Symposium on the Economics of Consumer Protection, Federalism and the Preemption of State Public Nuisance Actions (November 2019).
•Speaker, BridgeportEnvironmental Class Action Webinar (March 2016).
•Speaker, Harris Martin Porter Ranch Gas Leak Litigation Conference, Testing of the Air Quality and Expert Witnesses for the Cases (January 19, 2016).
•Daniel P. Mensher, With Friends Like These... The Trouble With Auer Deference, 43 Envtl. Law Rev. 4 (2013).
•Speaker, Oregon Water Law Conference, Addressing issues in Water Quality Trading (November 7, 2013).
•Speaker, Northwest Environmental Conference and Tradeshow, The Precautionary Principle in Environmental Law (December 11, 2013).
•Speaker, RainOps Conference, Spokane, WA, Longview, WA, Clean Water Act stormwater regulation (2013).
•Presenter, Oregon State Bar Environmental and Natural Resources Committee annual Continuing Legal Education Program, Salmon issues in Oregon and the Pacific Northwest, (2013).
•Speaker, Oregon State Bar brown bag CLE debate with Oregon DOJ assistant attorney general about the Supreme Court case Decker v. NEDC (2012).
•Daniel P. Mensher, Common Law On Ice: Using Federal Nuisance Law to Address Global Warming, 37 Envtl. Law Rev. 2 (2007).
•Chris Rycewicz and Dan Mensher, Growing State Authority Under the Clean Water Act, 22 Nat. Resources & Env’t 2 (2007).
Honors & Awards
•The Best Lawyers in America, Seattle, Washington, 2026:
•Mass Tort Litigation / Class Actions - Plaintiffs
•Selected to Rising Stars list in Super Lawyers - Washington, 2025
•Lawdragon 500 Leading Environmental Lawyers - The Green 500, 2025-2026
