Following amendment to the Kansas Constitution in 1986, the county remained a prohibition, or "dry", county until 2004, when voters approved the sale of alcoholic liquor by the individual drink with a 30% food sales requirement.
Coffey County's population was estimated to be 8,701 in the year 2006, a decrease of 179, or −2.0%, over the previous six years.
As of the U.S. Census in 2000, there were 8,865 people, 3,489 households, and 2,477 families residing in the county. The population density was 14 people per square mile (5/km²). There were 3,876 housing units at an average density of 6 per square mile (2/km²). The racial makeup of the county was 96.95% White, 0.25% Black or African American, 0.52% Native American, 0.34% Asian, 0.01% Pacific Islander, 0.50% from other races, and 1.43% from two or more races. Hispanic or Latino of any race were 1.55% of the population.
There were 3,489 households out of which 33.20% had children under the age of 18 living with them, 60.70% were married couples living together, 6.90% had a female householder with no husband present, and 29.00% were non-families. 26.00% of all households were made up of individuals and 12.60% had someone living alone who was 65 years of age or older. The average household size was 2.49 and the average family size was 2.99.
In the county the population was spread out with 26.80% under the age of 18, 6.50% from 18 to 24, 26.40% from 25 to 44, 24.00% from 45 to 64, and 16.20% who were 65 years of age or older. The median age was 39 years. For every 100 females there were 96.20 males. For every 100 females age 18 and over, there were 92.50 males.
The median income for a household in the county was $37,839, and the median income for a family was $44,912. Males had a median income of $31,356 versus $20,666 for females. The per capita income for the county was $18,337. About 5.00% of families and 6.60% of the population were below the poverty line, including 5.00% of those under age 18 and 9.80% of those age 65 or over.
Coffey County is divided into fourteen townships. The city of Burlington is considered governmentally independent and is excluded from the census figures for the townships. In the following table, the population center is the largest city (or cities) included in that township's population total, if it is of a significant size.