The terrain in Madison County consists of gently rolling terrain, sloped to the east-southeast, largely devoted to agriculture. The Elkhorn River runs eastward across the upper central portion of the county. The county has a total area of, of which is land and is water.
As of the 2000 United States Census, there were 35,226 people, 13,436 households, and 8,894 families in the county. The population density was 62 people per square mile. There were 14,432 housing units at an average density of 25 per square mile. The racial makeup of the county was 91.35% White, 0.94% Black or African American, 1.19% Native American, 0.40% Asian, 0.03% Pacific Islander, 5.06% from other races, and 1.03% from two or more races. 8.64% of the population were Hispanic or Latino of any race. There were 13,436 households out of which 33.20% had children under the age of 18 living with them, 54.70% were married couplesliving together, 8.40% had a female householder with no husband present, and 33.80% were non-families. 27.90% of all households were made up of individuals and 12.40% had someone living alone who was 65 years of age or older. The average household size was 2.52 and the average family size was 3.12. The county population contained 26.80% under the age of 18, 11.60% from 18 to 24, 27.10% from 25 to 44, 20.10% from 45 to 64, and 14.40% who were 65 years of age or older. The median age was 35 years. For every 100 females, there were 98.50 males. For every 100 females age 18 and over, there were 94.80 males. The median income for a household in the county was $35,807, and the median income for a family was $45,073. Males had a median income of $30,631 versus $21,343 for females. The per capita income for the county was $16,804. About 7.50% of families and 11.20% of the population were below the poverty line, including 13.00% of those under age 18 and 11.50% of those age 65 or over.
Madison County voters have been strongly Republican for many decades, voting for the Republican candidate in every presidential election except for three from 1900 onward. In addition, no Democratic presidential candidate has won the county since 1936.
