|— City —|
|• Mayor||Ron Counts|
|• Total||11.94 sq mi (30.92 km2)|
|• Land||11.58 sq mi (29.99 km2)|
|• Water||0.36 sq mi (0.93 km2)|
|Elevation||605 ft (184 m)|
|• Estimate (2011)||20,881|
|• Density||1,796.9/sq mi (693.8/km2)|
|Time zone||Central (CST) (UTC-6)|
|• Summer (DST)||CDT (UTC-5)|
|GNIS feature ID||0713393|
Arnold is located at . According to the United States Census Bureau, the city has a total area of 11.94 square miles (30.92 km2), of which, 11.58 square miles (29.99 km2) is land and 0.36 square miles (0.93 km2) is water.(38.432753, -90.369393)
|This section requires expansion. (November 2009)|
The first European settler in Arnold was Jean Baptiste Gamache, who operated a ferry boat across the Meramec River in exchange for 1050 arpents of land granted by the King of Spain. This ferry was on the King's Trace or El Camino Real, (royal road), from St. Louis, Missouri, to Ste. Genevieve, Missouri.
Politics and Government 
Eminent Domain 
Arnold was a focal point over the eminent domain issue in Missouri. In January 2004, the City of Arnold announced that THF Realty had approached them regarding developing a section of Arnold known as the Triangle, an area bordered by Route 141, Interstate 55, and Church Road in the city limits. The city voted in favor of the Triangle Development project proceeding on September 16, 2005.
According to an agreement with THF, Arnold would acquire the properties in the triangle and would be reimbursed its costs by THF. Arnold offered the property owners a buy-out, and most accepted. Some businesses were promised relocation  either in the new development or elsewhere in the city. Others were not given this option. Some refused the offer, and the city moved to condemn the dissenting properties. One such hold out was Homer R. Tourkakis, the owner of a dental practice on the corner of the triangle formed by Route 141 and Interstate 55.
Tourkakis claims that THF never made a fair-market offer. THF claims they offered to rebuild his practice elsewhere and purchase his property for $600,000. Tourkakis's property was declared blighted, and Arnold sought to seize it under eminent domain. Tourkakis fought these proceedings in the Jefferson County courts. Arnold and THF argued that although incorporated cities aren't explicitly granted the use of eminent domain does not imply they are denied it. The judge ruled that because Arnold is a third-class city, under Missouri law, it cannot use eminent domain to seize properties.
Red Light Cameras 
2010 census 
As of the census of 2010, there were 20,808 people, 8,090 households, and 5,695 families residing in the city. The population density was 1,796.9 inhabitants per square mile (693.8 /km2). There were 8,547 housing units at an average density of 738.1 per square mile (285.0 /km2). The racial makeup of the city was 96.3% White, 0.6% African American, 0.2% Native American, 0.9% Asian, 0.6% from other races, and 1.4% from two or more races. Hispanic or Latino of any race were 2.2% of the population.
There were 8,090 households out of which 34.1% had children under the age of 18 living with them, 52.3% were married couples living together, 12.9% had a female householder with no husband present, 5.2% had a male householder with no wife present, and 29.6% were non-families. 23.9% of all households were made up of individuals and 9.7% had someone living alone who was 65 years of age or older. The average household size was 2.55 and the average family size was 3.02.
The median age in the city was 39.2 years. 23.3% of residents were under the age of 18; 8.6% were between the ages of 18 and 24; 26.3% were from 25 to 44; 27.4% were from 45 to 64; and 14.4% were 65 years of age or older. The gender makeup of the city was 48.7% male and 51.3% female.
2000 census 
As of the census of 2000, there were 19,965 people, 7,550 households, and 5,564 families residing in the city. The population density was 1,775.0 inhabitants per square mile (685.2/km²). There were 7,913 housing units at an average density of 703.5 per square mile (271.6/km²). The racial makeup of the city was 97.91% White, 0.30% African American, 0.21% Native American, 0.38% Asian, 0.02% Pacific Islander, 0.27% from other races, and 0.92% from two or more races. Hispanic or Latino of any race were 1.04% of the population.
There were 7,550 households out of which 34.3% had children under the age of 18 living with them, 58.2% were married couples living together, 11.1% had a female householder with no husband present, and 26.3% were non-families. 22.0% of all households were made up of individuals and 7.7% 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 3.05.
In the city the population was spread out with 25.5% under the age of 18, 8.4% from 18 to 24, 30.4% from 25 to 44, 23.6% from 45 to 64, and 12.1% who were 65 years of age or older. The median age was 37 years. For every 100 females there were 94.7 males. For every 100 females age 18 and over, there were 90.3 males.
The median income for a household in the city was $47,188, and the median income for a family was $53,664. Males had a median income of $37,972 versus $27,222 for females. The per capita income for the city was $20,378. About 3.0% of families and 4.4% of the population were below the poverty line, including 6.1% of those under age 18 and 3.6% of those age 65 or over.
There are currently two high schools in Arnold, Missouri: Fox High School and Seckman Senior High School in the Fox C-6 School District. There are 4 middle schools, Fox Middle School, Seckman Middle School, Antonia Middle School, and Ridgewood Middle School. There are 11 Elementary schools, Antonia Elementary, Clyde Hamrick, Fox, George Guffey, Lone Dell, Meramec Heights,Raymond and Nancy Hodge, Rockport, Seckman, and Sherwood. There is one pre-school, Don Earl.
- "US Gazetteer files 2010". United States Census Bureau. Retrieved 2012-07-08.
- "American FactFinder". United States Census Bureau. Retrieved 2012-07-08.
- "Population Estimates". United States Census Bureau. Retrieved 2013-01-04.
- "American FactFinder". United States Census Bureau. Retrieved 2008-01-31.
- "US Board on Geographic Names". United States Geological Survey. 2007-10-25. Retrieved 2008-01-31.
- "US Gazetteer files: 2010, 2000, and 1990". United States Census Bureau. 2011-02-12. Retrieved 2011-04-23.
-  Case Summary for January 17, 2008
-  Homer Tourkakis: Victim of Eminent Domain Abuse
- Lynn Horsley, Caught Red Handed? Although Red-Light Cameras Haven't Gotten an OK in Kansas, Some Missouri Cities Are Installing Them, THE KANSAS CITY STAR, Apr. 1, 2006, p. A1
- Hoekstra v. City of Arnold, No. 4:08CV0267, 2009 WL 259857 (E.D. Mo. Feb. 3, 2009)