This county was established March 5, 1853, and took its name from the Blue Earth River, for a bluish green earth that was used by the Sisseton Dakota as a pigment, found in a shaley layer of the rock bluff of this stream about three miles from its mouth.

The blue earth was the incentive and cause of a very interesting chapter of our earliest history. Pierre Charles Le Sueur, the French explorer, before his first return to France in 1695, had discovered the locality whence the Indians procured this blue and green paint, which he thought to be an ore of copper, and he then took some of it to Paris, submitted it to L'Huillier, one of the king's assayers, and secured the royal commission to work the mines. But disasters and obstacles deterred him from this project until four years later, when having come from a third visit in France with 30 miners to Biloxi, near the mouth of the Mississippi, he ascended this river in the year 1700, using a sailing and rowing vessel and two canoes. Coming forward along the Minnesota River, he reached the mouth of the Blue Earth River on the last day in September or the first in October.

Le Sueur spent the ensuing year on this river, having built a camp or post named Fort L'Huillier, and in the spring mined a large quantity of the supposed copper ore. Taking a selected portion of the ore, amounting to two tons, and leaving a garrison at the fort, Le Sueur again navigated nearly the whole length of the Mississippi and arrived at the Gulf of Mexico in February 1702. Thence with Pierre Le Moyne, Sieur d'Iberville, the founder and first governor of Louisiana, who was a cousin of Le Sueur's wife, he sailed for France in the latter part of April, carrying the ore or blue earth, of which, however, nothing more is known.

Thomas Hughes of Mankato, historian of the city and county, identified in 1904 the sites of Fort L'Huillier and the mine of the blue or green earth, which are described in a paper contributed to the Minnesota Historical Society Collections (vol. XII, pp. 283-85).

André Pénicaut's Relation of Le Sueur's expedition was translated by Alfred J. Hill in the Minnesota Historical Society Collections (vol. III, 1880, pp. 1-12), and a map showing the locations of the fort and mine, ascertained by Hughes, was published in 1911 by Newton H. Winchell, on p. 493, The Aborigines of Minnesota (1911). From that expedition and the mine, we have the name of the Blue Earth River and of this county, and also of the township and city of Blue Earth in Faribault County.

This name was probably received by Le Sueur and his party from that earlier given to the river by the Dakota. The Relation of Pénicaut, however, might be thought to indicate otherwise, as follows: "We called this Green river, because it is of that color by reason of a green earth which, loosening itself from the copper mines, becomes dissolved in it and makes it green." In the language of the Dakota the same word, to, is used both for blue and green, and their name of the Blue Earth River is Makato (maka, earth; to, blue or green). William H. Keating wrote in the Narrative of Stephen H. Long's expedition, 1823: "By the Dacotas it is called Makato Osa Watapa, which signifies 'the river where blue earth is gathered.' "

The Dakota name is retained, with slight change, by the township and city of Mankato. On the earliest map of Minnesota Territory, in 1850, it appeared as Mahkahta for one of its original nine counties, reaching from the Mississippi above the Crow Wing west to the Missouri.