According to the U.S. Census Bureau, the county has a total area of, of which is land and is water. The southwest corner of Marion County is the intersection of the Baseline with the Third Principal Meridian, the point of origin for the third survey of the Northwest Territory under the Land Ordinance of 1785. The origin is marked with a boulder south of Centralia just off U.S. 51.
Climate and weather
In recent years, average temperatures in the county seat of Salem have ranged from a low of in January to a high of in July, although a record low of was recorded in January 1994 and a record high of was recorded in August 1983. Average monthly precipitation ranged from in January to in May.
As of the 2010 United States Census, there were 39,437 people, 16,148 households, and 10,746 families residing in the county. The population density was. There were 18,296 housing units at an average density of. The racial makeup of the county was 93.1% white, 3.9% black or African American, 0.6% Asian, 0.3% American Indian, 0.4% from other races, and 1.6% from two or more races. Those of Hispanic or Latino origin made up 1.4% of the population. In terms of ancestry, 27.5% were German, 15.8% were Irish, 13.6% were English, and 10.8% were American. Of the 16,148 households, 30.9% had children under the age of 18 living with them, 48.9% were married couplesliving together, 12.8% had a female householder with no husband present, 33.5% were non-families, and 28.8% of all households were made up of individuals. The average household size was 2.40 and the average family size was 2.91. The median age was 41.4 years. The median income for a household in the county was $38,974 and the median income for a family was $50,518. Males had a median income of $41,428 versus $28,042 for females. The per capita income for the county was $20,493. About 12.2% of families and 16.5% of the population were below the poverty line, including 23.9% of those under age 18 and 9.6% of those age 65 or over.
Initially a strongly Democratic anti-Yankee county, Marion County has undergone two transitions. Between 1912 and 2004 it was a perfect bellwether apart from the Catholicism-influenced 1960 election when substantial anti-Catholic voting by its largely southern white population caused it to support Republican Richard Nixon. Since the beginning of the twenty-first century, strong opposition to the Democratic Party's liberal views on social issues has transformed the county into a powerfully Republican one, with Hillary Clinton receiving a vote share over twelve percent smaller than any pre-2010 Democratic presidential nominee.
