Cass County is a county located in the U.S. state of Iowa. As of the 2020 census, the population was 13,127. Its county seat is Atlantic. It was named to honor Lewis Cass, who was the 1848 Democratic nominee for president.

Cass County is named in honor of Lewis Cass, a Michigan senator and an unsuccessful Democratic candidate for the presidency in 1848. The county was established within its present boundaries in 1851 and originated in 1853. Religious persecution was responsible for bringing the first people of European ancestry to Cass County. The Mormons, fleeing from Illinois, were the earliest settlers, and established a community at Indiantown in 1846.

At Indiantown, two of the three commissioners selected to locate a county seat were chosen. The site they chose was one mile (1.6 km) from Indiantown and named Lewis. Most of the people and businesses in Indian Town moved to Lewis shortly after it was laid out. In 1856, a frame courthouse was built, and eight years later a small stone building was completed for the county treasurer's office. In 1857, there was an attempt to relocate the county seat to Grove City; it was unsuccessful. On October 20, 1869, after a due canvass of the vote on the re-location of the county seat, the Board declared the city of Atlantic the county seat and ordered the county officers to that place.

In 1872, the first courthouse built in Atlantic was completed. Until it was completed county offices were held in various empty buildings. Ten years later the county built its second courthouse at Atlantic. The $65,000 building was destroyed by a fire in 1932. The fire started in the clock tower and gutted all of the second floor. Most of the county records and equipment were saved.

The present, fourth, courthouse was completed in 1934. While it was being built, county offices were located in the Atlantic Motors building, where an attempted robbery of the treasurer's safe took place. The robbers were interrupted and escaped, but without any money. The final cost of the fourth courthouse was $130,000: $65,000 came from a county bond issue, $46,500 from insurance and the remaining was covered by a Public Works Administration federal grant. The concrete and brick building is three stories high. It was dedicated on December 26, 1934, with the main speaker being the Governor of Iowa, Clyde L. Herring. The Board of Supervisors approved a new jail addition, which was dedicated in 1984.

According to the U.S. Census Bureau, the county has a total area of 565 square miles (1,460 km), of which 564 square miles (1,460 km) is land and 0.7 square miles (1.8 km) (0.1%) is water.

• Audubon County (north)
• Adair County (east)
• Adams County (southeast)
• Montgomery County (southwest)
• Pottawattamie County (west)
• Shelby County (northwest)

Due to its proximity to Cass County, Nebraska, and because both of those counties receive most of their broadcasts from Omaha, Nebraska, references to 'Cass County' must be frequently disambiguated, or result in confusion.

Historical population

Census	Pop.	Note	%±
1860	1,612		—
1870	5,464		239.0%
1880	16,943		210.1%
1890	19,642		15.9%
1900	21,274		8.3%
1910	19,047		−10.5%
1920	19,421		2.0%
1930	19,422		0.0%
1940	18,647		−4.0%
1950	18,532		−0.6%
1960	17,919		−3.3%
1970	17,007		−5.1%
1980	16,932		−0.4%
1990	15,128		−10.7%
2000	14,684		−2.9%
2010	13,956		−5.0%
2020	13,127		−5.9%
2023 (est.)	13,130		0.0%
U.S. Decennial Census 1790-1960 1900-1990 1990-2000 2010-2018

The 2020 census recorded a population of 13,127 in the county, with a population density of 23.2868/sq mi (8.9911/km). 96.75% of the population reported being of one race. 92.23% were non-Hispanic White, 0.22% were Black, 2.33% were Hispanic, 0.19% were Native American, 0.27% were Asian, 0.59% were Native Hawaiian or Pacific Islander and 4.17% were some other race or more than one race. There were 6,398 housing units of which 5,818 were occupied.

The 2010 census recorded a population of 13,956 in the county, with a population density of 24.745/sq mi (9.554/km). There were 6,591 housing units, of which 5,980 were occupied.

As of the census of 2000, there were 14,684 people, 6,120 households, and 4,094 families residing in the county. The population density was 26 people per square mile (10 people/km). There were 6,590 housing units at an average density of 12 units per square mile (4.6 units/km). The racial makeup of the county was 98.84% White, 0.21% Black or African American, 0.12% Native American, 0.14% Asian, 0.04% Pacific Islander, 0.31% from other races, and 0.34% from two or more races. 0.69% of the population were Hispanic or Latino of any race.

There were 6,120 households, out of which 29.30% had children under the age of 18 living with them, 56.60% were married couples living together, 7.20% had a female householder with no husband present, and 33.10% were non-families. 29.80% of all households were made up of individuals, and 15.90% had someone living alone who was 65 years of age or older. The average household size was 2.32 and the average family size was 2.87.

In the county, the population was spread out, with 23.80% under the age of 18, 6.80% from 18 to 24, 24.80% from 25 to 44, 23.80% from 45 to 64, and 20.80% who were 65 years of age or older. The median age was 42 years. For every 100 females there were 94.30 males. For every 100 females age 18 and over, there were 90.20 males.

The median income for a household in the county was $32,922, and the median income for a family was $40,564. Males had a median income of $29,736 versus $20,108 for females. The per capita income for the county was $17,067. About 7.20% of families and 11.10% of the population were below the poverty line, including 14.40% of those under age 18 and 10.10% of those age 65 or over.

• Edwin Perkins, inventor of Kool-Aid
• Ed Podolak, a former running back for Kansas City Chiefs
• Earl Caddock, a champion amateur and pro wrestler of the 1910s and early 1920s.
• Steve H. Hanke, professor of applied economics at Johns Hopkins University, adviser to presidents, currency reformer, and commodity and currency trader

Cass County is divided into sixteen townships:

The population ranking of the following table is based on the 2020 census of Cass County. † county seat

Rank	City/Town/etc.	Municipal type	Population (2020 Census)
1	† Atlantic	City	6,792
2	Griswold	City	994
3	Anita	City	963
4	Massena	City	359
5	Lewis	City	357
6	Cumberland	City	251
7	Marne	City	110
8	Wiota	City	91

Cass County has historically been one of the most solidly Republican counties in Iowa. The only Democrat ever to win the county was Franklin D. Roosevelt in his 1932 landslide, taking the county by less than 2%. Despite Roosevelt winning by an even larger landslide nationally just four years later, he lost Cass County by more than 13%.

United States presidential election results for Cass County, Iowa

Year	Republican		Democratic		Third party(ies)
	No.	%	No.	%	No.	%
2024	5,007	69.38%	2,077	28.78%	133	1.84%
2020	4,969	68.29%	2,201	30.25%	106	1.46%
2016	4,761	66.82%	1,951	27.38%	413	5.80%
2012	4,217	58.53%	2,858	39.67%	130	1.80%
2008	4,006	54.49%	3,211	43.68%	135	1.84%
2004	4,796	63.58%	2,679	35.52%	68	0.90%
2000	4,206	61.13%	2,481	36.06%	193	2.81%
1996	3,384	49.29%	2,616	38.11%	865	12.60%
1992	3,176	45.00%	2,231	31.61%	1,651	23.39%
1988	3,962	56.86%	2,934	42.11%	72	1.03%
1984	5,053	67.10%	2,417	32.09%	61	0.81%
1980	5,391	66.34%	2,176	26.78%	559	6.88%
1976	4,589	60.79%	2,866	37.97%	94	1.25%
1972	5,234	72.06%	1,923	26.48%	106	1.46%
1968	5,223	67.30%	2,136	27.52%	402	5.18%
1964	4,182	51.01%	4,006	48.87%	10	0.12%
1960	6,290	67.24%	3,059	32.70%	5	0.05%
1956	6,103	68.34%	2,818	31.56%	9	0.10%
1952	7,355	75.61%	2,349	24.15%	23	0.24%
1948	5,106	59.66%	3,372	39.40%	81	0.95%
1944	5,610	65.53%	2,928	34.20%	23	0.27%
1940	6,377	62.69%	3,763	36.99%	32	0.31%
1936	5,622	56.43%	4,284	43.00%	56	0.56%
1932	4,215	48.76%	4,339	50.19%	91	1.05%
1928	6,120	69.43%	2,640	29.95%	55	0.62%
1924	5,721	68.70%	1,099	13.20%	1,508	18.11%
1920	6,558	78.23%	1,668	19.90%	157	1.87%
1916	2,763	59.75%	1,801	38.95%	60	1.30%
1912	1,724	38.35%	1,510	33.59%	1,261	28.05%
1908	2,799	61.19%	1,655	36.18%	120	2.62%
1904	3,050	65.55%	1,394	29.96%	209	4.49%
1900	3,128	60.12%	2,010	38.63%	65	1.25%
1896	2,959	56.11%	2,240	42.47%	75	1.42%

• National Register of Historic Places listings in Cass County, Iowa
• The Cass County Courthouse Article

• County website

41°19′46″N 94°55′45″W / 41.32944°N 94.92917°W / 41.32944; -94.92917