According to the U.S. Census Bureau, the county has a total area of, of which is land and is water. Clay County is home to the large Barringer Slough wetland.
The Spencer Municipal Airport is located three miles northwest of the business district of Spencer. It provides service to the county and surrounding communities.
The 2010 census recorded a population of 16,667 in the county, with a population density of. There were 8,062 housing units, of which 7,282 were occupied.
2000 census
As of the census of 2000, there were 17,372 people, 7,259 households, and 4,776 families residing in the county. The population density was 30 people per square mile. There were 7,828 housing units at an average density of 14 per square mile. The racial makeup of the county was 98.08% White, 0.17% Black or African American, 0.10% Native American, 0.82% Asian, 0.03% Pacific Islander, 0.25% from other races, and 0.55% from two or more races. 1.13% of the population were Hispanic or Latino of any race. There were 7,259 households out of which 30.70% had children under the age of 18 living with them, 55.60% were married couplesliving together, 6.80% had a female householder with no husband present, and 34.20% were non-families. 29.80% of all households were made up of individuals and 13.70% had someone living alone who was 65 years of age or older. The average household size was 2.35 and the average family size was 2.92. In the county, the population was spread out with 24.70% under the age of 18, 8.00% from 18 to 24, 26.90% from 25 to 44, 22.50% from 45 to 64, and 18.00% who were 65 years of age or older. The median age was 39 years. For every 100 females, there were 93.30 males. For every 100 females age 18 and over, there were 90.90 males. The median income for a household in the county was $35,799, and the median income for a family was $42,769. Males had a median income of $30,163 versus $21,068 for females. The per capita income for the county was $19,451. About 6.30% of families and 8.20% of the population were below the poverty line, including 11.10% of those under age 18 and 8.10% of those age 65 or over.
Economy
The county has been based on farming. The 1980s farm crisis caused some families to have to give up their farms, and farms have been merged to industrial scale. The population has declined since 1980. At one point Great Lakes Airlines was headquartered in Summit Township, Clay County.
The population ranking of the following table is based on the 2010 census of Clay County. †county seat
Rank
City/Town/etc.
Municipal type
Population
1
†Spencer
City
11,233
2
Everly
City
603
3
Royal
City
446
4
Peterson
City
334
5
Fostoria
City
231
6
Dickens
City
185
7
Webb
City
141
8
Greenville
City
75
9
Rossie
City
70
10
Gillett Grove
City
49
Politics
For most of its history, Clay County has primarily voted for Republican party candidates in presidential elections. Exceptions to this include Bull Moose candidate & former president Theodore Roosevelt winning a majority in 1912, the longest Democratic streak in its history when Franklin D. Roosevelt & Harry S. Truman were on the ballot from 1932 to 1948, Lyndon B. Johnson winning in a landslide statewide & nationally in 1964, Michael Dukakis getting a huge boost statewide thank to the Midwest farm crisis in 1988, & Bill Clinton benefiting from Ross Perot gaining significant minorities of the county's vote in 1992 & 1996. The level of Republican support has increased significantly in recent years, with Hillary Clinton posting the worst performance since 1952 when Adlai Stevenson won an identical percentage of the county's votes at 26.1%.
