The 2010 census recorded a population of 16,843 in the county, with a population density of. There were 7,594 housing units, of which 6,846 were occupied.
As of the census of 2000, there were 16,181 people, 6,649 households, and 4,281 families residing in the county. The population density was 37 people per square mile. There were 7,241 housing units at an average density of 17 per square mile. The racial makeup of the county was 96.02% White, 0.64% Black or African American, 0.17% Native American, 1.70% Asian, 0.04% Pacific Islander, 0.53% from other races, and 0.91% from two or more races. 1.84% of the population were Hispanic or Latino of any race. There were 6,649 households out of which 31.10% had children under the age of 18 living with them, 53.10% were married couplesliving together, 8.00% had a female householder with no husband present, and 35.60% were non-families. 30.40% of all households were made up of individuals and 10.70% had someone living alone who was 65 years of age or older. The average household size was 2.34 and the average family size was 2.93. In the county, the population was spread out with 24.40% under the age of 18, 7.60% from 18 to 24, 24.40% from 25 to 44, 29.80% from 45 to 64, and 13.80% who were 65 years of age or older. The median age was 41 years. For every 100 females, there were 95.90 males. For every 100 females age 18 and over, there were 93.60 males. The median income for a household in the county was $33,851, and the median income for a family was $43,819. Males had a median income of $32,066 versus $22,479 for females. The per capita income for the county was $19,579. About 7.40% of families and 10.90% of the population were below the poverty line, including 12.40% of those under age 18 and 9.00% of those age 65 or over.
Law and government
Jefferson County's executive branch is a three-member board of supervisors who are elected to four-year terms. The current board members, their party affiliations and the years in which they were last elected are: Steve Burgmeier , Lee Dimmitt, and Richard Reed. Other elected officials are Scott Reneker, county auditor; Gregg Morton, county sheriff; Terri Kness, county treasurer; and Kelly Spees, county recorder.
Politics
Historically, Jefferson County heavily favored presidential candidates from the Republican Party. It is noted for being one of the few counties in Iowa that never voted for Woodrow Wilson, along with having never given a Democrat who carried the county over sixty percent of the vote. In recent decades, politics within the county have become more competitive. From 1992-2012, the county has been carried by the Democratic presidential candidate in every election with the exception of 2000, when George W. Bush obtained a plurality. The last Republican to win the county with a majority of the vote was Ronald Reagan in 1984. In the 1996 presidential election, Jefferson County was the only county in the United States to give any candidate less than forty percent of the vote, with Bill Clinton winning the county 35.1% to Bob Dole's 34.4%. In 2016, Donald Trump received a plurality of the votes in Jefferson County.
