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Mark Sheridan

Senior Counsel

555 17th Street, Suite 3200, Denver, CO 80202

Mark guides clients through the complexities of commercial litigation.

With more than 30 years of experience, he understands the needs of his clients who are facing the possibility of a trial.

His background includes trial, appellate, arbitration and alternative dispute resolution experience, including extensive work in antitrust, breach of contract, real property, land use, oil and gas, water rights, Indian law and litigation arising out of administrative regulation of business.

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Client Results

Representative Cases
  • Appellate counsel to diversified energy company in series of federal appeals, including a petition for writ of certiorari in the U. S. Supreme Court, concerning royalty accounting for oil and gas production on Indian land
  • Defense of national title insurer in complex insurance bad faith claim and business tort action
  • Defense of privately held real estate developer in multiple lawsuits brought by citizen group to block development of corporate office campus based on zoning regulations and restrictive covenants
  • Lead counsel for eight defendants in federal securities fraud action arising out of sale of privately held business in outside advertising industry to private equity fund
  • Representation of a multi-national natural resources company in complex state corporate income tax matter, contesting state tax assessment on foreign source income and disallowance of tax deductions
  • Representation of City of Santa Fe in complex declaratory judgment action to determine contract rights of major real estate development to use city groundwater wells and obligations to comply with city water use restrictions
  • Trial on behalf of hundreds of non-Indian water rights owners of major federal Indian water rights case, adjudicating surface and groundwater claims of Pueblo Indians
  • Representation of major publicly-held corporate defendants in media and health care industries in consumer class action antitrust and unfair trade practices litigation
  • Trial on behalf of oil and gas producers of complex breach of contract action against public utility, obtaining $31.9 million federal court jury verdict for plaintiffs
  • Trial on behalf of United States of antitrust suit under Section 7 of the Clayton Act to block merger in mining industry



Bar Admissions


  • Chambers USA, Native American Law, 2012, 2013
  • The Best Lawyers in America® Litigation – Antitrust, 2003-2023
  • Southwest Super Lawyers® 2007, 2010-2016

Professional and Civic Affiliations

  • Member, State Bar of New Mexico
  • Member, American Bar Association

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