He represented Rainy River in the Legislative Assembly of Ontario initially as a Liberal-Conservative in 1911, and then as a Conservative from 1914 to 1923 and 1926 to 1929.

[10][11] He was instrumental in pioneering the use of mechanized equipment in the woods (while other firms were still relying mainly on horses), as well as in using aircraft.

[11] He set up an educational foundation to provide financial assistance for students from the area,[12] for which he was honoured in 1958 by the Ontario Secondary School Teachers Federation.

[2] Mathieu also established a wildlife reserve and help fund community projects in the region.

[12] During his time in the provincial assembly, he helped promote the development of roads to encourage settlement in the district.