Marion is a city in and the county seat of Williamson County, Illinois, United States. The population was 17,193 at the 2010 census. It is part of a dispersed urban area that developed out of the early 20th-century coal fields.
Marion is in central Williamson County, with a narrow strip of city limits extending south beyond Creal Springs to the valley of Sugar Creek in Johnson County. Marion is 44 miles (71 km) south of Mount Vernon, Illinois, and 57 miles (92 km) north of Paducah, Kentucky. Carbondale is 17 miles (27 km) to the west, and Harrisburg is 22 miles (35 km) to the east.
According to the 2010 census, Marion has a total area of 16.217 square miles (42.00 km), of which 15.99 square miles (41.41 km) (or 98.6%) is land and 0.227 square miles (0.59 km) (or 1.4%) is water.
As of the census of 2000, there were 16,035 people, 6,902 households, and 4,341 families residing in the city. The population density was 1,250.2 people per square mile (482.6/km²). There are 7,555 housing units at an average density of 589.0 per square mile (227.4/km²). The racial makeup of the city was 92.89% White, 4.34% African American, 0.24% Native American, 0.83% Asian, 0.05% Pacific Islander, 0.44% from other races, and 1.21% from two or more races. Hispanic or Latino of any race were 1.60% of the population.
There were 6,902 households out of which 28.1% had children under the age of 18 living with them, 47.0% were married couples living together, 12.5% had a female householder with no husband present, and 37.1% were non-families. 33.1% of all households were made up of individuals and 16.4% had someone living alone who was 65 years of age or older. The average household size was 2.25 and the average family size was 2.86.
In the city the population was spread out with 22.8% under the age of 18, 8.3% from 18 to 24, 26.5% from 25 to 44, 22.5% from 45 to 64, and 20.0% who were 65 years of age or older. The median age was 40 years. For every 100 females, there were 87.9 males. For every 100 females age 18 and over, there were 84.4 males.
The median income for a household in the city was $30,364, and the median income for a family was $39,275. Males had a median income of $31,520 versus $22,609 for females. The per capita income for the city was $19,073. About 11.2% of families and 14.9% of the population were living below the poverty line, including 22.9% of those under the age of 18 and 10.6% of those 65 and older.