The town is inside the boundaries of the Baie-James Municipality and is the second largest Cree community with a population of 3,731 people in 2021.

The surface area of the town is 807.75 square kilometres (311.87 sq mi) (Category I land, as defined in the James Bay and Northern Quebec Agreement).

Cree have lived in the Rupert River watershed area and around Lake Mistassini for centuries.

The travel route shifted to the south - first through Oskelaneo when the railroad was built in 1910, and subsequently through the Lac Saint-Jean area.

Through time Mistissini and the various posts in the area were also known as "Maison Dorval", "Patagoosh", "Abatagoushe", "Mistassini", and "Baie-du-Poste".