According to the U.S. Census Bureau, the county has a total area of 664 square miles (1,720 km2), of which 657 square miles (1,700 km2) is land and 7.7 square miles (20 km2) (1.2%) is water. The county's eastern border is the Mississippi River.
As of the 2020 Census Jefferson County had a population of 226,739. The ethnic and racial makeup of the county was 90.4% white (89.7% non-Hispanic white), 1.0% black or African American, 0.3% American Indian, 0.8% Asian, 0.8% some other race, and 6.6% two or more races. 2.4% of the population was of Hispanic or Latino origin.
As of the 2010 Census Jefferson County had a population of 218,733. The reported ethnic and racial make up of the population was 95.4% non-Hispanic white, 0.8% African-American, 0.3% Native American, 0.6% Asian, 0.4% reporting some other race, 1.3% reporting two or more races and 1.6% Hispanic or Latino of any race.
There were 71,499 households, out of which 38.90% had children under the age of 18 living with them, 61.00% were married couples living together, 10.40% had a female householder with no husband present, and 23.70% were non-families. 18.90% of all households were made up of individuals, and 6.10% had someone living alone who was 65 years of age or older. The average household size was 2.74 and the average family size was 3.12.
In the county, the population was spread out, with 27.90% under the age of 18, 8.50% from 18 to 24, 31.80% from 25 to 44, 22.50% from 45 to 64, and 9.20% who were 65 years of age or older. The median age was 35 years. For every 100 females there were 98.90 males. For every 100 females age 18 and over, there were 95.90 males.
The median income for a household in the county was $60,636, and the median income for a family was $66,697. Males had a median income of $37,822 versus $25,440 for females. The per capita income for the county was $25,058. About 4.90% of families and 6.80% of the population were below the poverty line, including 8.10% of those under age 18 and 6.30% of those age 65 or over.
There were 146,316 registered voters in 2008. As of Oct. 24, 2012, there were 148,011.
Historically, the Democratic Party has controlled politics at the local level in Jefferson County but starting in 2010 the Republican Party has flipped the county with 66% of the population voting Republican in 2020.
A predominantly suburban county, Jefferson County used to be fairly independent-leaning at the federal level with a tendency to tilt Democratic. Presidential elections in Jefferson County were often very close; George W. Bush just narrowly carried the county in 2004 by less than 600 votes and by just over a half of a percentage point. Al Gore and Barack Obama also just narrowly carried the county in 2000 and 2008, respectively. Bill Clinton, however, did manage to carry Jefferson County by double digits both times in 1992 and 1996. However, in 2012 the county began to swing hard to the right with Mitt Romney carrying it with 55% of the vote. In 2016 Donald Trump won the county with 65% of the vote, the largest margin of any candidate since Lyndon Johnson in 1964.
Typical of the suburban culture in most counties throughout the country, voters in Jefferson County tend to be rather centrist on social issues but more liberal on economic issues. In 2004, Missourians voted on a constitutional amendment to define marriage as the union between a man and a woman—it overwhelmingly passed Jefferson County with 72.56 percent of the vote. The initiative passed the state with 71 percent of support from voters as Missouri became the first state to ban same-sex marriage. In 2006, Missourians voted on a constitutional amendment to fund and legalize embryonic stem cell research in the state—it narrowly passed Jefferson County with 51.85 percent voting for the measure. The initiative narrowly passed the state with 51 percent of support from voters as Missouri became one of the first states in the nation to approve embryonic stem cell research. In 2006, Missourians voted on a proposition (Proposition B) to increase the minimum wage in the state to $6.50 an hour—it passed Jefferson County with 79.90 percent of the vote. The proposition strongly passed every single county in Missouri with 78.99 percent voting in favor as the minimum wage was increased to $6.50 an hour in the state. During the same election, voters in five other states also strongly approved increases in the minimum wage.
Clinton had a large initial lead in Missouri at the beginning of the evening as the rural precincts began to report, leading several news organizations to call the state for her; however, Obama rallied from behind as the heavily African American precincts from St. Louis began to report and eventually put him over the top. In the end, Obama received 49.32 percent of the vote to Clinton's 47.90% — a 1.42% difference. Both candidates split Missouri's 72 delegates, as the Democratic Party utilizes proportional representation.
Hillary Rodham Clinton received more votes, a total of 19,075, than any candidate from either party in Jefferson County during the 2008 Missouri Presidential Primaries. She also received more votes than the total number of votes cast in the entire Republican Primary in Jefferson County.
According to a 2012 census study by the University of Wisconsin Population Health Institute and the Robert Wood Johnson Foundation, Jefferson County led the Saint Louis metropolitan area in the number of adults who smoke, roughly 30% of all adult residents of Jefferson County smoked or used tobacco in some form, compared to the 19% national average and the 24% Missouri state average.
Jefferson County and the state of Missouri led the nation in methamphetamine production, peaking in the mid-2000s.