Allen County's population was estimated to be 13,677 in the year 2006, a decrease of 701, or -4.9%, over the previous six years.
As of the U.S. Census in 2000, there were 14,385 people, 5,775 households, and 3,892 families residing in Allen County. The population density was 29 people per square mile (11/km²). There were 6,449 housing units at an average density of 13 per square mile (5/km²). The racial makeup of the county was 94.80% White, 1.63% Black or African American, 0.78% Native American, 0.26% Asian, 0.86% from other races, and 1.68% from two or more races. Hispanic or Latino of any race were 1.93% of the population. 28.8% were of German, 20.3% American, 9.8% English and 8.6% Irish ancestry according to Census 2000.
There were 5,775 households out of which 29.80% had children under the age of 18 living with them, 54.60% were married couples living together, 8.90% had a female householder with no husband present, and 32.60% were non-families. 28.50% of all households were made up of individuals and 14.40% had someone living alone who was 65 years of age or older. The average household size was 2.43 and the average family size was 2.98.
In the county the population was spread out with 25.20% under the age of 18, 9.80% from 18 to 24, 24.10% from 25 to 44, 22.90% from 45 to 64, and 18.00% who were 65 years of age or older. The median age was 39 years. For every 100 females there were 95.60 males. For every 100 females age 18 and over, there were 90.70 males.
The median income for a household in the county was $31,481, and the median income for a family was $39,117. Males had a median income of $27,305 versus $19,221 for females. The per capita income for the county was $15,640. About 11.30% of families and 14.90% of the population were below the poverty line, including 20.20% of those under age 18 and 12.70% of those age 65 or over.
The first white inhabitants located in Allen County during the early part of the year 1855. Settlement progressed rapidly during the spring and summer with the greater number of settlers located along the Neosho River. Although many of the early settlers were pro-slavery men, few slaves were brought into the county. The freestate men showed such open antagonism toward slaveholders, that the slaves were soon given their freedom or taken from the county by their masters.
The last year of the territorial period (1860) was one of the hardest because it was the year of the great drought. The population of the county was about 3,000, and with such a scanty crop, the prospect of starvation seemed imminent. Most of the people had come into the county within two years and with starvation and hardship before them, returned to the east.
During the years of the American Civil War the country developed, but slowly. From 1865 to 1870 there was a steady increase, the population then numbering 7,022. For the next three years the country was settled rapidly, and numerous improvements were made, as well as thousands of acres of land brought under cultivation. This period was perhaps the most progressive one in the history of the county; money was plenty and nearly every one did business, or bought property to the full extent of his capital. The result was that with the financial panic of 1873, followed by the "grasshopper raid" of 1874, nearly all improvement stopped, value of property depreciated, and many of the settlers (nearly one-third) left the county. In 1875 the population numbered 6,638. The next year times began to look better, and by 1878 the population was 8,954. With the increasing prosperity of the country, the population numbered 10,436 in 1881, while improvements that were made kept pace with the settlement. In 1882 the population had increased to 11,098.
The population of the county continued to grow until it finally peaked at 27,640 with the census of 1910. In recent decades, the population decline has leveled off.
U.S. Route 54 is an east/west route passing through (from east to west) the cities of Moran, La Harpe, Gas, and Iola (the county seat). Outside the county the route connects to Fort Scott in the east and Yates Center and eventually Wichita in the west. Passing through eastern portions of the county and the cities of Mildred and Moran, U.S. Route 59 is one of two north/south routes. It connects to Kincaid and eventually Ottawa and Lawrence in the north and Erie in the south. The other route is U.S. Route 169 which passes through western portions of the county and bypasses to the east of the cities of Iola, Bassett, and Humboldt. It connects to Chanute and eventually Coffeyville in the south. The segment between Iola and Chanute is a freeway with fully-controlled access, although there is only one lane in each direction. US-169 provides a direct route for traveling between Kansas City and Tulsa, Oklahoma.