Mathieu's practice focuses on major infrastructure and energy projects. He has international experience advising public and private entities on projects in various industries in Canada, the United Kingdom and Australia. These industries range from renewable energy and carbon capture to transportation (metro systems, rails and roads) and natural resources (oil/gas and mining).

Mathieu also has specific expertise related to climate change, particularly greenhouse-gas-emission regulatory requirements and carbon-pricing mechanisms. In this respect, he was a member of the delegation of the Republic of Palau that negotiated the adoption of the Paris Agreement on climate change. He also advised the U.K. Department for Energy Security and Net Zero on the deployment of its large-scale Carbon Capture, Usage and Storage and clean-hydrogen subsidy programs.

Before rejoining Blakes, where he began his career, Mathieu worked for nearly five years in Sydney, Australia, and London, England, for an international law firm, where he advised the respective governments on the procurement of major transportation, renewable energy and carbon capture infrastructures.

Mathieu is fluent in English and French.