Harlan was platted in 1858. It was named for one of Iowa's early U.S. Senators, James Harlan. Harlan was designated county seat in 1859. The town was incorporated on May 2, 1879.
At the 2010 census there were 5,106 people, 2,222 households, and 1,341 families living in the city. The population density was. There were 2,410 housing units at an average density of. The racial makup of the city was 97.0% White, 0.5% African American, 0.4% Native American, 0.6% Asian, 0.6% from other races, and 0.9% from two or more races. Hispanic or Latino of any race were 1.9%. Of the 2,222 households 26.7% had children under the age of 18 living with them, 47.8% were married couples living together, 9.7% had a female householder with no husband present, 2.8% had a male householder with no wife present, and 39.6% were non-families. 34.2% of households were one person and 17.7% were one person aged 65 or older. The average household size was 2.26 and the average family size was 2.88. The median age was 43.8 years. 23.8% of residents were under the age of 18; 6.1% were between the ages of 18 and 24; 21.4% were from 25 to 44; 27% were from 45 to 64; and 21.8% were 65 or older. The gender makeup of the city was 46.4% male and 53.6% female.
At the 2000 census there were 5,282 people, 2,204 households, and 1,498 families living in the city. The population density was 1,209.8 people per square mile. There were 2,306 housing units at an average density of 528.2 per square mile. The racial makup of the city was 98.30% White, 0.08% African American, 0.30% Native American, 0.47% Asian, 0.17% from other races, and 0.68% from two or more races. Hispanic or Latino of any race were 0.62%. Of the 2,204 households 31.1% included children under the age of 18; 56.6% were married couples living together, 8.9% consisted of a female householder with no husband present, and 32.0% were non-families. 28.9% of households were one person and 16.3% were one person aged 65 or older. The average household size was 2.35, and the average family size was 2.88. 25.1% are under the age of 18, 6.1% from 18 to 24, 24.2% from 25 to 44, 22.4% from 45 to 64, and 22.3% 65 or older. The median age was 41 years. For every 100 females, there were 90.9 males. For every 100 females age 18 and over, there were 85.4 males. The median household income was $35,899 and the median family income was $45,888. Males had a median income of $31,365 versus $19,671 for females. The per capita income for the city was $17,514. About 5.3% of families and 7.0% of the population were below the poverty line, including 7.3% of those under age 18 and 9.4% of those age 65 or over.
