Lyon County is the most northwesterly county of the U.S. state of Iowa. As of the 2020 census, the population was 11,934. The county seat is Rock Rapids.

Lyon County is named in honor of Brigadier General Nathaniel Lyon, who served in the Mexican–American War and the Civil War. He was killed at the Battle of Wilson's Creek, Missouri, on August 10, 1861, after which the county was named for him. The county's name was originally Buncombe County, but was changed by the state legislature on September 11, 1862.

The land that makes up Lyon County was ceded to the federal government by the Sioux Native Tribe through a treaty signed on July 23, 1851. The boundaries of the county were set on January 15, 1851, and attached to Woodbury County (then called Wahkaw County) for administration purposes. Lyon County was split from Woodbury County on January 1, 1872.

The first non-indigenous resident to live in Lyon County was Daniel McLaren, known as "Uncle Dan". He lived near the Sioux River for a short time, spending his time hunting and trapping. He moved out of the county early in its settlement to stake a claim further west. The second settler in the area was known as "Old Tom", a hunter and trapper who lived briefly near present-day Rock Rapids. While setting his traps, Old Tom was killed by Sioux tribespeople.

In 1862–1863, a group of men from the east coast spent time in the county on a hunting trip. They were: Roy McGregor, George Clark and Thomas Lockhart. During the winter, Lockhart and McGregor were hunting elk along the Little Rock creek and encountered a group of Sioux tribespeople. Lockhart was killed by an arrow, but McGregor was able to escape and rejoin Clark. The two continued to hunt and trap until March 1863. During a spring flood, Clark was drowned and McGregor decided to move back east.

The first permanent settlement in Lyon County was built by Lewis P. Hyde in July 1866. The county's population reached 100 persons in 1869, entirely through migration and settlement. The first non-indigenous child born in the county was Odena Lee, born on May 28, 1871. The first election in the county was held on October 10, 1871, and recorded 97 votes.

According to the U.S. Census Bureau, the county has a total area of 588 square miles (1,520 km), of which 588 square miles (1,520 km) is land and 0.1 square miles (0.26 km) (0.02%) is water.

Lyon County is the location of Gitchie Manitou State Preserve, which contains some of the oldest exposed bedrock in the country.

Lake Pahoja is located in the northwest part of the county. It is a man-made lake with an area of just over 28 hectares (69 acres).

• U.S. Highway 18
• U.S. Highway 75
• Iowa Highway 9
• Iowa Highway 182

• Rock County, Minnesota (north)
• Nobles County, Minnesota (northeast)
• Osceola County (east)
• Sioux County (south)
• Lincoln County, South Dakota (west)
• Minnehaha County, South Dakota (northwest)

Historical population

Census	Pop.	Note	%±
1870	221		—
1880	1,968		790.5%
1890	8,680		341.1%
1900	13,165		51.7%
1910	14,624		11.1%
1920	15,431		5.5%
1930	15,293		−0.9%
1940	15,374		0.5%
1950	14,697		−4.4%
1960	14,468		−1.6%
1970	13,340		−7.8%
1980	12,896		−3.3%
1990	11,952		−7.3%
2000	11,763		−1.6%
2010	11,581		−1.5%
2020	11,934		3.0%
2023 (est.)	12,324		3.3%
U.S. Decennial Census 1790-1960 1900-1990 1990-2000 2010-2018

The 2020 census recorded a population of 11,934 in the county, with a population density of 20.3348/sq mi (7.8513/km). 97.19% of the population reported being of one race. 91.03% were non-Hispanic White, 0.34% were Black, 3.41% were Hispanic, 0.27% were Native American, 0.20% were Asian, 0.39% were Native Hawaiian or Pacific Islander and 4.37% were some other race or more than one race. There were 4,817 housing units, of which 4,466 were occupied.

The 2010 census recorded a population of 11,581 in the county, with a population density of 19.7123/sq mi (7.6110/km). There were 4,848 housing units, of which 4,442 were occupied.

As of the census of 2000, there were 11,763 people, 4,428 households, and 3,263 families residing in the county. The population density was 20 people per square mile (7.7 people/km). There were 4,758 housing units at an average density of 8 units per square mile (3.1 units/km). The racial makeup of the county was 99.13% White, 0.09% Black or African American, 0.14% Native American, 0.15% Asian, 0.01% Pacific Islander, 0.10% from other races, and 0.37% from two or more races. 0.36% of the population were Hispanic or Latino of any race.

There were 4,428 households, out of which 34.80% had children under the age of 18 living with them, 67.10% were married couples living together, 4.40% had a female householder with no husband present, and 26.30% were non-families. 24.30% 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 2.61 and the average family size was 3.13.

In the county, the population was spread out, with 28.00% under the age of 18, 7.60% from 18 to 24, 24.60% from 25 to 44, 20.90% from 45 to 64, and 18.80% who were 65 years of age or older. The median age was 38 years. For every 100 females there were 98.40 males. For every 100 females age 18 and over, there were 94.60 males.

The median income for a household in the county was $36,878, and the median income for a family was $45,144. Males had a median income of $29,462 versus $19,385 for females. The per capita income for the county was $16,081. About 4.90% of families and 7.00% of the population were below the poverty line, including 7.90% of those under age 18 and 10.30% of those age 65 or over.

The population ranking of the following table is based on the 2020 census of Lyon County.

† county seat

Rank	City/Town/etc.	Municipal type	Population (2020 Census)
1	† Rock Rapids	City	2,611
2	George	City	1,077
3	Inwood	City	928
4	Larchwood	City	926
5	Doon	City	619
6	Little Rock	City	439
7	Lester	City	296
8	Alvord	City	206

Lyon County is among the most GOP-friendly counties in Iowa. Only two Democrats have ever won the county: Grover Cleveland by a twenty-vote plurality in 1892, and Franklin D. Roosevelt in his 1932 and 1936 landslides. Though Roosevelt carried the county by a very comfortable margin in both elections, the county went back to its solid Republican roots in 1940 and has not been won by a Democrat since, with Michael Dukakis being the last Democrat to even reach 30%. The only other times someone besides a Republican won this county were in 1912 when Theodore Roosevelt carried it in the split election, and in 1924 when Robert M. La Follette carried it by a narrow plurality. Even so, both of these candidates were affiliated with the Republican Party outside of these elections.

United States presidential election results for Lyon County, Iowa

Year	Republican		Democratic		Third party(ies)
	No.	%	No.	%	No.	%
2024	5,899	84.25%	1,023	14.61%	80	1.14%
2020	5,707	83.16%	1,067	15.55%	89	1.30%
2016	5,192	81.42%	920	14.43%	265	4.16%
2012	4,978	76.48%	1,423	21.86%	108	1.66%
2008	4,471	71.88%	1,675	26.93%	74	1.19%
2004	4,751	77.87%	1,303	21.36%	47	0.77%
2000	3,918	73.34%	1,313	24.58%	111	2.08%
1996	3,396	63.45%	1,489	27.82%	467	8.73%
1992	3,272	57.14%	1,331	23.24%	1,123	19.61%
1988	3,517	66.81%	1,706	32.41%	41	0.78%
1984	4,178	74.05%	1,401	24.83%	63	1.12%
1980	4,349	70.05%	1,431	23.05%	428	6.89%
1976	3,558	64.06%	1,870	33.67%	126	2.27%
1972	3,788	72.12%	1,407	26.79%	57	1.09%
1968	4,195	72.91%	1,403	24.38%	156	2.71%
1964	3,185	53.66%	2,747	46.28%	3	0.05%
1960	4,917	73.65%	1,752	26.24%	7	0.10%
1956	4,356	70.83%	1,790	29.11%	4	0.07%
1952	4,893	78.59%	1,324	21.27%	9	0.14%
1948	2,500	52.74%	2,174	45.86%	66	1.39%
1944	3,065	60.79%	1,970	39.07%	7	0.14%
1940	3,880	59.33%	2,648	40.49%	12	0.18%
1936	2,264	38.09%	3,590	60.40%	90	1.51%
1932	1,684	31.79%	3,543	66.89%	70	1.32%
1928	3,170	65.52%	1,632	33.73%	36	0.74%
1924	2,082	43.99%	481	10.16%	2,170	45.85%
1920	3,633	81.48%	729	16.35%	97	2.18%
1916	1,760	59.78%	1,137	38.62%	47	1.60%
1912	412	14.84%	896	32.27%	1,469	52.90%
1908	1,650	58.93%	1,064	38.00%	86	3.07%
1904	1,802	65.05%	841	30.36%	127	4.58%
1900	1,666	55.00%	1,289	42.56%	74	2.44%
1896	1,568	51.33%	1,464	47.92%	23	0.75%

• National Register of Historic Places listings in Lyon County, Iowa
• Blood Run Site

• Smith, Paul C.; Colby, Lucy Jo (1975). Buncombe to "Twenty-Two". Lyon County Reporter.
• Andreas, Alfred Theodore (1875). Illustrated historical atlas of the state of Iowa. Chicago: Andreas Atlas Company.
• Ogle, G.A. (1905). Compendium of History, Reminiscence and Biography of Lyon County, Iowa.

• Collected articles dealing with early Lyon County history
• Iowa State Association of Counties page about Lyon County
• Informational page about Lake Pahoja

43°23′N 96°13′W / 43.383°N 96.217°W / 43.383; -96.217