El Dorado, founded by Matthew Rainey, is a city in, and the county seat of, Union County, on the southern border of Arkansas, United States. According to the 2010 census, the population of the city is 18,884. The city was the heart of the 1920s oil boom in the area. During World War II, it became a center of the chemical industry, which still plays a part in the economy, as do oil and timber.