Blue Earth County is a county in the State of Minnesota. As of the 2020 census, the population was 69,112. Its county seat is Mankato. The county is named for the Blue Earth River and for the deposits of blue-green clay once evident along the banks of the Blue Earth River. Blue Earth County is part of the Mankato-North Mankato metropolitan area.

Mankato is a city in Blue Earth, Nicollet, and Le Sueur counties in the U.S. state of Minnesota. It is the county seat of Blue Earth County, Minnesota. The population was 44,488 according to the 2020 census, making it the 21st-largest city in Minnesota, and the 5th-largest outside of the Minneapolis–Saint Paul metropolitan area. It is along a large bend of the Minnesota River at its confluence with the Blue Earth River. Mankato is across the Minnesota River from North Mankato. Mankato and North Mankato have a combined population of 58,763 according to the 2020 census. It completely encompasses the town of Skyline. North of Mankato Regional Airport, a tiny non-contiguous part of the city lies within Le Sueur County. Most of the city is in Blue Earth County.