This article is about the Illinois city in Saline County. For the Illinois township in McDonough County, see Eldorado Township, McDonough County, Illinois. Eldorado is a city in Saline County, Illinois, United States. The population was 4,122 at the 2010 census, with a 1925 peak of 8,000. Although the city's name is spelled as if it were Spanish, the name was originally "Elder-Reado" -- a combination of the last names of the town's two founders, Judge Samuel Elder and Joseph Read.