In the 2021 Census of Population conducted by Statistics Canada, Saint-Marc-du-Lac-Long had a population of 365 living in 178 of its 263 total private dwellings, a change of -8.1% from its 2016 population of 397. With a land area of 148.64 km2 (57.39 sq mi), it had a population density of 2.5/km2 (6.4/sq mi) in 2021.[4]