Mathieu grew up in Ottawa and lived in Sandy Hill and Lowertown neighbourhoods for the majority of his life. He is a graduate of the University of Ottawa, where he completed a Masters degree in sports management. After completing his studies, Mathieu decided to take a leap into municipal politics. In 2010, he won the municipal election for the Rideau-Vanier ward and became the youngest Councillor at the City Council table at the age of 25. Since coming to City Hall, Mathieu has worked for the betterment and beautification of the National Capital and the Rideau Vanier Ward, which is home to Lowertown, Sandy Hill and Vanier.
In addition to his work at City Council, Mathieu is an active member of a number of committees: he is the chair the Ottawa Community Housing Board, member of the Transportation Committee and Community and Protective Services Committee, board member of Ottawa Public Health and Ottawa Sports Council, and co-chair of the City's Task Force on Canada's 150th Anniversary in 2017.