Mary Paterson

Mary Paterson

Partner, Litigation

Contact Information

tel: 416.862.4924



Bar Admission

Ontario, 2005


  • Osgoode Hall Law School, LL.B.
  • Osgoode Hall Trial Advocacy Course
  • University of Calgary, B.Comm.
  • University of Calgary, B.A. (English)



Mary positions her clients to achieve positive settlements or succeed at trial. Although based in Ontario, with many appearances before the Ontario Superior Court, including the Commercial List, she has also conducted trials in the Tax Court of Canada, the Federal Court of Canada, the Alberta Court of Queen’s Bench, the British Columbia Supreme Court, and the Supreme Court of Newfoundland and Labrador. She has appeared on motions in the Manitoba Court of Queen’s Bench.

Mary has also appeared on appeals in the Supreme Court of Canada, the Ontario Court of Appeal, the Ontario Divisional Court, the Alberta Court of Appeal, the Quebec Court of Appeal, and the British Columbia Court of Appeal.

Her areas of specialty include tax litigation, insolvency work (receiverships, bankruptcies and proceedings pursuant to the Companies’ Creditors Arrangement Act) and contract disputes and other general commercial litigation.

Mary has been recognized by multiple organizations, including in the 2022 edition of Best Lawyers Canada, as a 2019 Lexpert Rising Star, the 2021 and 2020 Benchmark Litigation 40 & Under Hot List, and as an Acritas 2018 Star Lawyer. She was also recognized by Osler associates in 2019 as an Osler Star Advisor for her work mentoring associates.

Mary is a member of the Board of Directors for Pro Bono Ontario. Throughout her career she worked to establish several pro bono programs within the firm and has appeared in the Court of Appeal on a pro bono basis numerous times. She won a 2011 Lexpert Zenith Award for her work assisting a UN employee obtain a widower’s benefit after his same-sex partner died while working for the UN.

Mary is certified in “The Path – Your Journey through Indigenous Canada”, an online program offered by the CBA. This program serves as an introduction to the history of Indigenous peoples and their relationship with European settlers, the British Crown and Dominion of Canada.

Before joining Osler, Mary clerked at the Court of Appeal for Ontario.

    • Multiple Canadian taxpayers in a dispute with the CRA regarding a tax shelter (trial)
    • Several trustees in disputes with the CRA regarding the residence of the trust, including a trial and appeal
    • A Canadian company in a dispute with the CRA regarding FAPI
    • A global remediation company defending breach of contract litigation
    • An accounting firm in an alleged wrongful dismissal case (trial)
    • The Greater Toronto Airports Authority in action related to the termination of a contract and taxes
    • A Canadian corporation in a dissenting shareholder dispute (trial)
    • A Canadian bank in a shareholder dispute
    • An individual in a shareholder dispute
    • Several Canadian franchisors in franchise disputes
    • An international company in a patent damages reference (Federal Court)
    • A Canadian company seeking payment from its insurer (Federal Court - trial)
    • Several Canadian Trustees in bankruptcy
    • Several Canadian court-appointed receivers and monitors
    • Several international corporations preparing for proceedings in the United States as well as in Canada under the Companies’ Creditors Arrangement Act
    • A Canadian secured creditor seeking to appoint a receiver

  • The Canadian Legal Lexpert Directory: Recognized in Litigation - Corporate Tax
  • Lexpert Rising Stars: Recognized as a Lexpert Rising Star: Leading Lawyers Under 40
  • Benchmark Litigation Canada: Recognized in Bankruptcy; Arbitration; Dispute Resolution; Commercial; Class Action (Future Star)
  • Best Lawyers in Canada: Recognized in Appellate Practice; Corporate and Commercial Litigation
  • Euromoney LMG: Recognized in Americas Rising Star Awards: Best in Litigation (Shortlisted)
  • Osler 2019 Toronto Advisor Award: Recognized as an outstanding partner that has gone above and beyond in providing advice, support, mentoring and encouragement to associate mentees

  • The Advocates’ Society

    Speaker, A Litigator’s Guide to Case Preparation, April 11, 2023

  • The Advocates’ Society, Tax Litigation: The Effective Witness

    Speaker, Effective Examinations and Cross-Examinations, January 18, 2023

  • Intensive Trial Advocacy Workshop (ITAW) 2022

    Co-Presenter, July 11 - 16, 2022

  • Litigation Contract Disputes - The Advocates’ Society

    Speaker, Gathering the Evidence at Discovery to Win, Toronto, June 18, 2020

  • The Advocates’ Society

    Speaker, Examining and Cross-examining Experts, Toronto, May 29, 2019

  • Insolvency Litigation - The Advocates’ Society

    Speaker, Competing Receivership and CCAA Applications, Toronto, April 12, 2019

  • Focus on Credibility - The Advocates’ Society

    Speaker, Keys to being a Credible Witness, Toronto, November 21, 2018

  • Osler Pride Network

    Speaker, Student Outreach & Networking Reception, July 10, 2018

  • Litigating Contract Disputes – The Advocates’ Society

    Speaker, Gather the Evidence at Discovery to Win, June 21, 2018

  • OBA's Institute

    Speaker, How to Thrive as a Young Lawyer in Today’s Market, Toronto, February 6 - 10, 2018

  • Settlement in a Tax Controversy (Dispute to Litigation)

    Speaker, October 1, 2017

  • Winner of the 2011 Lexpert Zenith Award for Pro Bono by Associates
  • Project Work, Board Member
  • Pro Bono Law Ontario, Board Member
  • Pro Bono Law Ontario, Court of Appeal Amicus Program (in person and virtual)
  • Pro Bono Law Ontario, Law Help Centre
  • Osler Community Law Committee
  • Outside counsel assisting the United Nations Office of Staff Legal Assistance and representing UN employees directly

  • Law Society of Ontario
  • The Advocates’ Society (member of Young Advocates’ Standing Committee, 2007-2010)
  • Young Canadian Arbitration Practitioners, Secretary
  • The Osgoode Society
  • Pro Bono Ontario
  • Project Work

  • Judicial Notice in the Misinformation Age, The Advocates' Journal, Fall 2023.
  • Apportionment of Damages in Negligence and Contract, Toronto Law Journal, January 2023.
  • The Business Benefits of Pro Bono: Not Just for the Public Good, The Advocates' Journal, Fall 2022.
  • Magnotta v. Yu: Navigating the Intricacies of Rule 49 Settlement Offers, Toronto Law Journal, April 2021.
  • H.M.B. Holdings: Clarifying the Limits of Reciprocal Enforcement of Judgments in Canada, Toronto Law Journal, January 2021.
  • When to Speak Up: Applying the Parol Evidence Rule in Contractual Disputes, Toronto Law Journal, February 2020.
  • Your First Civil Trial 2016: Practical Tips for Trial Preparation, OBA, March 2016.
  • 1196303 Inc.v. Glen Grove Suites Inc.: Using Privity and Agency to Hold Third Parties Liable, Toronto Law Journal, March 2016.
  • Arbitral Award Not Enforced Because Service Was Ineffective, The Advocates’ Society Communities; Arbitration and Mediation, January 2016.
  • Sattva Capital Corp. v. Creston Moly Corp.: Are Issues of Contractual Interpretation Certifiable in Class Proceedings as Common Issues?, Bank & Finance Law Review, April 2015.
  • Supreme Court Focuses on Getting to the Merits, Colloque National Sur Les Recours Collectifs – Editions Yvon Blais, March 26, 2015.
  • Procedural Tips for Trial Preparation, OBA Young Lawyers Division: Your First Civil Trial Conference, March 6, 2015.
  • Boilerplate: Tips and Tricks on Drafting, Osler, March 4, 2015.
  • Missed in Translation: Bringing Your U.S. Franchise Agreement to Canada, LJN’s Franchising Business & Law Alert Vol. 21, No. 2, November 2014.
  • 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, November 2014.
  • Court Rejects Class Action Settlement As Substantively Unfair, Osler’s Canadian Class Action Defence Blog, March 26, 2014.
  • Staying a Third Party Claim: Is the Third Party Bound to Findings in the Common Issues Trial? Osler, Hoskin & Harcourt LLP, March 20, 2014.
  • OBA YLD Conference: Your First Civil Trial – Procedural Tips for Trial Preparation, March 7, 2014.
  • Does Access to Justice Trump Individual Issues? Osler, Hoskin & Harcourt LLP, March 6, 2014.
  • Just Enough: Practical Boundaries of Certification, Colloque National sur les Recours Collectifs, Barreau du Quebec, March 2014.
  • Tips on Drafting Arbitration Clauses, Osler Franchise Review.
  • Limitation Periods Will Continue to Run During the Appeal of a Bankruptcy Order, Weil Bankruptcy Blog.
  • Leave to Appeal Denied – Ontario Court of Appeal Reviews the Test for Leave to Appeal Decisions in CCAA Proceedings, Weil Bankruptcy Blog.
  • Exclusion Clauses Prevent Co-Defendants From Collecting Contribution and Indemnity, Canadian Bar Association National Civil Litigation Section Newsletter, July 2013 (Paul Ivanoff, Mary Paterson and Carly Fidler).
  • Fairness prevails: Ontario Court applies “fraud upon the bankruptcy law” principle to void provision of contract, Weil Bankruptcy Blog.
  • Law Commission of Ontario To Review Class Action Procedure,, January 8, 2014 (Mary Paterson and Christopher Naudie).
  • Arbitration Agreements Can Preclude Competition Claims  and Class Actions, Commercial Litigation, Volume XV, November 2013
  • No way out – Ontario insolvency court approves class action settlement that prohibits any affected person from opting out, Weil Bankruptcy Blog, May 13, 2013 (Mary Paterson and Patrick Riesterer) .
  • New York Fries: Court Awards $500,000 Against Franchisees for Defamation, Osler Franchise Review, April 2013 (Mary Paterson and Lia Bruschetta).
  • The Class of 2012: Where Are We Now?, Editions Yvon Blais, March 2013 (Mary Paterson, Sonia Bjorkquist and Karin Sachar).
  • Environmental Contamination and Regulatory Orders: Who Pays? Weil Bankruptcy Blog.
  • Multi-Jurisdictional Classes: Does Van Breda Change Anything?, OBA Class Act Vol. 3, No. 1, December 2012 (Mary Paterson and Sonia Bjorkquist).
  • Litigating a Cross-Border Class Action in Canada, ALM Cross-Border Litigation Forum in New York City, September, 2012 (Mary Paterson, Karin Sachar, Deborah Glendinning and Sonia Bjorkquist).
  • Ontario court of appeal upholds extraordinary DIP financing arrangements in CCAA proceedings over objections of majority creditors, National Insolvency Review, vol. 29 no. 4, August 2012 (Mary Paterson and Edward Sellers).
  • DIP Financing Rules Crystallized: Interim DIP Financing Arrangements Approved by the Ontario Court of Appeal, Weil Bankruptcy Blog.
  • A Cautionary Tale on Privilege Claims by Court-Appointed Receivers, OBA Insolvency News, Volume 28, No. 1, February 2012 (Mary Paterson and John A. MacDonald).
  • Baton Rouge: Juicy Legal Developments on Disclosure Obligations, Osler Franchise Review, March 2012 (Mary Paterson).
  • Multi-Jurisdictional Class Actions: Will the New CBA and ABA Protocols Solve All our Problems?, Développements Récents en Recours Collectifs, Volume 345, Barreau du Québec, March 2012 (Mary Paterson, Mike Eizenga and Sonia Bjorkquist).
  • Court Assigns Dealership Agreement over Ford’s Objections, Toronto Law Journal, January 2013 (Mary Paterson and John MacDonald).
  • Competence-Competence Does Not Apply to Parties Seeking Arbitration, YCAP, 2012 (Mary Paterson).
  • Courts Reject Technical Defences and Allow Rescission, Osler Franchise Review, October 2011 (Mary Paterson).
  • A Lying Bankrupt: A Trustee’s Obligation, Rebuilding Success, Fall/Winter 2011 (Mary Paterson and John MacDonald).
  • What to Do When a Franchisee Summons you to Testify, Osler Franchise Review, May 2011 (Mary Paterson).