Mattoon was the site of the "Mad Gasser" attacks of the 1940s.
As of the census of 2000, there were 18,291 people, 8,105 households, and 4,676 families residing in the city. The population density was 1,964.8 people per square mile (758.6/km²). There were 8,830 housing units at an average density of 948.5/sq mi (366.2/km²). The racial makeup of the city was 96.64% White, 1.42% African American, 0.18% Native American, 0.41% Asian, 0.01% Pacific Islander, 0.41% from other races, and 0.93% from two or more races. Hispanic or Latino of any race were 1.27% of the population.
There were 8,105 households out of which 26.7% had children under the age of 18 living with them, 43.6% were married couples living together, 10.5% had a female householder with no husband present, and 42.3% were non-families. 35.6% of all households were made up of individuals and 14.9% had someone living alone who was 65 years of age or older. The average household size was 2.21 and the average family size was 2.87.
In the city the population was spread out with 22.3% under the age of 18, 11.3% from 18 to 24, 27.5% from 25 to 44, 20.9% from 45 to 64, and 18.1% who were 65 years of age or older. The median age was 38 years. For every 100 females there were 89.9 males. For every 100 females age 18 and over, there were 85.8 males.
The median income for a household in the city was $31,800, and the median income for a family was $43,780. Males had a median income of $32,339 versus $21,949 for females. The per capita income for the city was $18,186. About 7.6% of families and 13.4% of the population were below the poverty line, including 15.5% of those under age 18 and 10.0% of those age 65 or over.
Transportation is still a vital part of local economic life. Much of the major commercial development in recent years has occurred along Interstate 57, which crosses the eastern edge of Mattoon.
Amtrak, the national passenger rail system, provides service to Mattoon. Amtrak Train 59, the southbound City of New Orleans, is scheduled to depart Mattoon at 11:13pm daily with service to Effingham, Centralia, Carbondale, Fulton, Newbern-Dyersburg, Memphis, Greenwood, Yazoo City, Jackson, Hazlehurst, Brookhaven, McComb, Hammond, and New Orleans. Amtrak Train 58, the northbound City of New Orleans, is scheduled to depart Mattoon at 5:23am daily with service to Champaign-Urbana, Kankakee, Homewood, and Chicago. Mattoon is also served by Amtrak Train 390/391, the Saluki, daily in the morning, and Amtrak Train 392/393, the Illini, daily in the afternoon/evening. Both the Saluki and the Illini operate between Chicago and Carbondale.
The Illinois Central Station in the heart of downtown Mattoon is badly decaying, and local activists have sought funding for years to restore it. 2.5 million dollars were obtained for it via the 2005 Transportation Bill. Today the station is unmanned; passengers boarding at the Mattoon station must order their tickets by telephone or online.