Monticello is located at . According to the United States Census Bureau, the city has a total area of, of which is land and is water.. Monticello is 823 feet above sea level. For many years, U.S. Route 151 passed directly through Monticello. In 2004, a four-lane bypass around Monticello was completed and opened. As a result, the highway was moved approximately one mile east of the previous route.
As of the census of 2010, there were 3,796 people, 1,693 households, and 991 families living in the city. The population density was. There were 1,839 housing units at an average density of. The racial makeup of the city was 98.6% White, 0.3% African American, 0.3% Asian, 0.3% from other races, and 0.5% from two or more races. Hispanic or Latino of any race were 1.3% of the population. There were 1,693 households of which 25.8% had children under the age of 18 living with them, 46.5% were married couples living together, 8.6% had a female householder with no husband present, 3.4% had a male householder with no wife present, and 41.5% were non-families. 35.6% of all households were made up of individuals and 18.1% had someone living alone who was 65 years of age or older. The average household size was 2.18 and the average family size was 2.82. The median age in the city was 43.3 years. 22.3% of residents were under the age of 18; 6.9% were between the ages of 18 and 24; 22.8% were from 25 to 44; 26.3% were from 45 to 64; and 21.8% were 65 years of age or older. The gender makeup of the city was 47.5% male and 52.5% female.
2000 census
As of the census of 2000, there were 3,607 people, 1,538 households, and 979 families living in the city. The population density was 980.8 people per square mile. There were 1,637 housing units at an average density of 445.1 per square mile. The racial makeup of the city was 98.75% White, 0.19% African American, 0.08% Native American, 0.14% Asian, 0.17% from other races, and 0.67% from two or more races. Hispanic or Latino of any race were 1.16% of the population. There were 1,538 households out of which 26.9% had children under the age of 18 living with them, 51.4% were married couples living together, 9.4% had a female householder with no husband present, and 36.3% were non-families. 31.5% of all households were made up of individuals and 17.4% had someone living alone who was 65 years of age or older. The average household size was 2.26 and the average family size was 2.82. Population spread: 23.1% under the age of 18, 7.3% from 18 to 24, 24.5% from 25 to 44, 21.3% from 45 to 64, and 23.8% who were 65 years of age or older. The median age was 42 years. For every 100 females, there were 85.0 males. For every 100 females age 18 and over, there were 82.2 males. The median income for a household in the city was $34,932, and the median income for a family was $42,616. Males had a median income of $30,324 versus $20,998 for females. The per capita income for the city was $16,699. About 3.6% of families and 7.2% of the population were below the poverty line, including 5.8% of those under age 18 and 6.7% of those age 65 or over.
