This county, established by legislative acts of March 3, 1855, and March 1, 1856, is named from theSt. Louis River, the largest entering Lake Superior, which flows through this county. The river was probably so named by Pierre Gaultier de Varennes, sieur de la Vérendrye (1685-1749), who was a very active explorer, in the years 1731 and onward, of the vast country from Pigeon River and Rainy Lake to the Saskatchewan and Missouri Rivers, establishing trading posts and missions. The king of France in 1749, shortly before the death of La Vérendrye, conferred on him the cross of St. Louis as a recognition of the importance of his discoveries, and thence the name of the St. Louis River appears to have come. On Jean Baptiste Louis Franquelin's map (1688) and Philippe Buache's map (1754), it is called the Riviére du Fond du Lac, and the map by Gilles Robert de Vaugondy (1755) and Jonathan Carver's map (1778) are the earliest to give the present name. St. Louis County has the distinction of being the largest county in this state, having an area of 6,611.75 square miles.

Saint Louis was born at Poissy, France, near Paris, April 25, 1215, and died near Tunis, Africa, August 25, 1270. From 1226 he was King Louis IX of France, his mother Blanche being regent during his minority. He undertook a crusade to the Holy Land in 1248, from which, after a terrible war, he returned to France in 1254. His second crusade was undertaken in 1267, for which he finally sailed from France on July 1, 1270, but in this expedition he died by an illness less than two months later. He is commemorated by the name of the city of St. Louis, but Louisiana was named for Louis XIV, who was king of France from 1643 to 1715.