Mille-Isles is in the Laurentian Hills, crossed by rivers and dotted with fish-filled lakes.

[4] The municipality is named after the old Mille-Isles Seigneury, which originally straddled the Mille Îles River (the seigneury uses the old spelling, whereas the river uses the modern word that substitutes a circumflex for the "s").

In 1683, the seigneury was granted to Michel-Sidrac Dugué de Boisbriand (circa 1638-1688), who was governor of Montreal in 1670.

In 1714, it was inherited by Charles-Gaspard Piot de Langloiserie (circa 1655-1715) and Jean Petit (1663-1720), husbands of Marie-Thérèse Dugué and Charlotte Dugué respectively, daughters of the first lord.

In 1752, additional land in the extreme north-west of the Mille-Isles Seigneury was given to Eustache Lambert Dumont and it is within this part that the municipality is located.