Dynamic Advocate and Advisor in High-Profile Public Interest & Business Law Matters
A strategic legal advisor and litigator with recognized experience in high-stakes public law, constitutional advocacy, and tax litigation, Guillaume Sirois brings a distinctive perspective shaped by his work on complex national matters. His background includes serving as counsel in two major national and one provincial commission of inquiry and managing sensitive legal files ranging from tax defence to challenges involving governmental authorities across Canada. Through strategic litigation and careful legal advisory, he helps clients protect their rights and interests when dealing with regulatory bodies and government institutions, delivering solutions that support business growth while safeguarding important public interests.
Guillaume’s advocacy focuses on consequential legal issues affecting governance, constitutional law, and human rights across Canada. His most significant work includes high-profile advocacy before national commissions of inquiry, where government transparency and accountability were central issues. At the Foreign Interference Commission, he cross-examined nearly one hundred witnesses, including the sitting Prime Minister, numerous ministers, and senior officials, regarding allegations of foreign interference and threats to democratic integrity. He also contributed his extensive knowledge to the Public Order Emergency Commission, which examined the first invocation of the Emergencies Act, conducting detailed legal analysis of the response by police forces (including the RCMP, OPP, and SQ) and municipalities such as Ottawa and Windsor during nationwide public order disturbances.
Guillaume applies this depth of procedural and constitutional experience across his broader practice. He has participated in litigation advancing claims under the Canadian Charter of Rights and Freedoms and Section 35 of the Constitution Act, 1982 on behalf of vulnerable communities across Canada. At the same time, he advises businesses on corporate structuring and represents clients in complex tax disputes. His professional background also includes service as a clerk at the Federal Court and international human rights work with Lawyers Without Borders in Ivory Coast, experiences that continue to inform his approach to advocacy and legal strategy.