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Mary  Paterson  - Osler, Hoskin & Harcourt LLP. Toronto , ON, CA

Mary Paterson Mary  Paterson

Partner, Litigation and Tax Litigation | Osler, Hoskin & Harcourt LLP

Toronto , ON, CANADA

Seasoned commercial litigator and trial lawyer with a practice in tax litigation, contract disputes, and insolvency proceedings.


Mary has appeared in the Ontario Court of Appeal, the Ontario Superior Court (including the Commercial List), the Federal Court, the Alberta Court of Queen’s Bench, the Manitoba Court of Queen’s Bench, the Supreme Court of Newfoundland and Labrador, and the Tax Court of Canada.

Industry Expertise (1)

Legal Services

Areas of Expertise (5)

Class Actions

Insolvency & Restructuring


Pro Bono

Tax Controversy & Litigation

Accomplishments (1)

Recognized as a Future Star by Benchmark Litigation Canada in 2015. (professional)

Benchmark Litigation Canada, 2015 Recognized for her pro bono work on behalf of a UN employee who obtained widower’s benefit after his same-sex partner died while working for the UN. Precedent setting case was recognized by Lexpert Magazine’s Zenith Award in 2011.

Education (3)

University of Calgary: B.Comm.

University of Calgary: B.A.

Osgoode Hall Law School: LL.B.

Affiliations (3)

  • Director Project Work
  • Law Society of Upper Canada
  • The Osgoode Society

Languages (1)

  • English

Media Appearances (4)

Osler lawyer’s efforts make traffic court disclosure the norm for self-represented litigants in Ontario

Financial Post  print


The pro bono efforts of Mary Paterson, a lawyer in Osler, Hoskin & Harcourt’s Toronto office, have been instrumental in making Crown disclosure a reality for self-represented litigants in Ontario’s traffic court.

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SCC issues ‘big change’ to contract law – Sattva gives last word to trial judges, arbitrators

Law Times  online


Arbitrators and trial judges should have the last word in most cases that turn on the interpretation of contracts, the Supreme Court of Canada has found.

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Turnaround Management Association Names Sale of Arctic Glacier as Transaction of the Year

Osler  online


We are proud to announce that the Turnaround Management Association (TMA) has awarded the Transaction of the Year, in the mid-size company category, to the sale of Arctic Glacier.

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Court of Appeal Overturns Pet Valu Decision: Class Members Can Openly Debate Opting Out Without Invalidating Opt-Out Period -

Financial Post  online


The decision in 1250264 Ontario Inc. v. Pet Valu Canada Inc. is important because it confirms that the deadline for opt-out periods is effectively carved in stone. It also grants dissident plaintiffs greater liberty in how they voice their opinions over the suit.

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Event Appearances (2)

Class Action Preparedness Brief - Multijurisdictional Matters (Speaker)

Osler Client Program, Class Actions  


Walking the Tightrope: How to Manage Risk Today to Prevent Tomorrow's Class Action (Speaker)

Osler Client Program  


Articles (10)

Tax disputes on the rise as government increases audit resources

Osler Year in Review


In response to mounting public concern over tax avoidance and evasion, the federal government in Budget 2016 announced the dedication of an additional $444.4 million to the Canada Revenue Agency (CRA) over the next five years to combat these issues. A Standing Committee on Finance was convened to study the CRA’s efforts in this regard. The federal funds are slated for hiring additional auditors and specialists and combatting criminal tax evasion, and are expected to generate an additional $2.6 billion in revenue for the federal government.

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Alberta court grants Encana’s application for a stay of action and referral to arbitration

Osler Update


On April 12, 2016, the Alberta Court of Queen’s Bench stayed an action started by Toyota Tsusho Wheatland Inc. (TTWI) against Encana Corporation (Encana) and referred the vast majority of the claims to arbitration. The Court’s decision, reported in Toyota Tsusho Wheatland Inc. v. Encana Corporation, 2016 ABQB 209, upholds the contractual autonomy of parties to have their disputes determined by mandatory arbitration, even where the arbitration touches on the interests of third parties not privy to the arbitration agreement.

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Your First Civil Trial 2016: Procedural Tips for Trial Preparation

Ontario Bar Association


At this half-day program, prominent litigators share with you their time-tested strategies and practical advice on topics ranging from preparing your case to the closing argument.

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1196303 Inc.v. Glen Grove Suites Inc.: Using Privity and Agency to Hold Third Parties Liable

Toronto Law Journal


The Ontario Court of Appeal’s August 2015 decision in 1196303 Inc. v. Glen Grove Suites Inc. held that a non-party to a settlement agreement, Glen Grove Suites Inc. (“Glen Grove”), was bound to obligations pursuant to that agreement.

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Ontario Court of Appeal upholds liability for tort of intentional interference with economic relations

Osler Update


The Ontario Court of Appeal has now emphasized that the tort of intentional interference with economic relations has narrow applicability, but does occur. In its March 2, 2016 decision in Grand Financial Management Inc. v. Solemio Transportation Inc., the Court considered a case in which one party (Grand Financial) threatened to put another (Solemio) out of business. Grand Financial also sought to disrupt Solemio’s business by influencing the actions of a third party (Arnold Bros.) and illegitimately seizing security interests held by another third party (RBC).

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Arbitral Award Not Enforced Because Service Was Ineffective

The Advocates’ Society Communities: Arbitration and Mediation


The Ontario Court of Appeal recently refused to allow enforcement of an arbitral award because the Notice of Application seeking enforcement was not properly served on a foreign state under the State Immunity Act (“SIA").

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Sattva Capital Corp. v. Creston Moly Corp.: Are Issues of Contractual Interpretation Certifiable in Class Proceedings as Common Issues?

Bank & Finance Law Review

Issues of contractual interpretation have been routinely certified as common issues in class proceedings, often with very little analysis.

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Supreme Court Focuses on Getting to the Merits

Colloque National Sur Les Recours Collectifs: Editions Yvon Blais


The Supreme Court of Canada's trilogy in late 2013 focused on getting to the merits of class proceedings more efficiently. This article considers how the courts in common law provinces implemented these directives in 2014, with a focus on evidentiary requirements, preferable procedure analysis and refinement of common issues. We also discuss how the efficiency principle is affecting procedural rulings in cross-border cases and driving increased use of summary procedures in class actions. We will continue to watch these developing trends in 2015.

Procedural Tips for Trial Preparation

OBA Young Lawyers Division: Your First Civil Trial Conference


Mary presented best practices for trial preparation checklists, witness interviews and preparation, requests to admit, Evidence Act requirements, and courtroom procedure. For copies of her paper, please contact her.

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Supreme Court of Canada Limits the Right to Appeal Commercial Arbitral Decisions: Contractual Interpretation Is a Question of Mixed Fact and Law

Commercial Litigation and Arbitration Review


On August 1, 2014, the Supreme Court of Canada (the “S.C.C.”) changed the landscape of arbitral and appellate advocacy in contractual disputes.

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