[3] The lake is fed by dozens of small rivers, including the Ashuapmushuan, the Mistassini, the Peribonka, the Des Aulnaies, the Métabetchouane, and the Ouiatchouane.

The lake was named Piekuakami by the Innu, the Indigenous people who occupied the area at the time of European arrival.

In the 20th century, pulp and paper mills and aluminum smelting rose to importance, encouraged by hydroelectric dams at Alma and on the Péribonka River.

The area is featured in the classic French-language novel Maria Chapdelaine by Louis Hémon published in 1914 and subsequently translated into twenty languages.

In the 1940s, during World War II, Lac Saint-Jean, along with various other regions within Canada, such as the Saguenay, Saint Helen's Island and Hull, Quebec, had prisoner-of-war camps.

It consists of fragments of island arcs and continental crust accreted to the south-eastern edge of Precambrian North American, Laurentia.

[1][6][7] Preserved within the down-faulted interior of the Saguenay Graben are two large eroded, isolated patches, known as outliers, of Paleozoic, Middle Ordovician, sedimentary rock composed of limestones and shales overlying Precambrian basement.

As a result, as the Laurentide Ice Sheet retreated, the Saguenay Graben was flooded by marine waters to form the Laflamme Sea.