As of the 2000 census, there were 74,798 people, 21,476 households, and 16,686 families living in the county. The population density was 14 people per square mile. There were 26,718 housing units at an average density of 5 per square mile. The racial makeup of the county was 74.72% Native American, 16.39% White, 0.46% Asian, 0.40% Black or African American, 0.04% Pacific Islander, 5.47% from other races, and 2.52% from two or more races. 12.40% of the population were Hispanic or Latino of any race. There were 21,476 households out of which 46.00% had children under the age of 18 living with them, 47.70% were married couplesliving together, 22.70% had a female householder with no husband present, and 22.30% were non-families. 19.50% of all households were made up of individuals and 5.30% had someone living alone who was 65 years of age or older. The average household size was 3.44 and the average family size was 3.99. In the county, the population was spread out with 38.00% under the age of 18, 9.70% from 18 to 24, 27.80% from 25 to 44, 17.60% from 45 to 64, and 6.90% who were 65 years of age or older. The median age was 27 years. For every 100 females there were 93.50 males. For every 100 females age 18 and over, there were 89.30 males. The median income for a household in the county was $25,005, and the median income for a family was $26,806. Males had a median income of $26,963 versus $21,014 for females. The per capita income for the county was $9,872. About 31.90% of families and 36.10% of the population were below the poverty line, including 42.30% of those under age 18 and 31.50% of those age 65 or over. The county's per-capita income makes it one of the poorest counties in the United States. McKinley County is one of only 38 county-level census divisions of the United States where the most spoken language is not English and one of only 3 where it is neither English nor Spanish. 45.75% of the population speak Navajoat home, followed by English at 38.87%, Zuñi at 9.03%, and Spanish at 5.72%.
2010 census
As of the 2010 census, there were 71,492 people, 21,968 households, and 16,219 families living in the county. The population density was. There were 25,813 housing units at an average density of. The racial makeup of the county was 75.5% Native American, 15.2% white, 0.8% Asian, 0.5% black or African American, 4.9% from other races, and 3.1% from two or more races. Those of Hispanic or Latino origin made up 13.3% of the population. Of the 21,968 households, 46.2% had children under the age of 18 living with them, 40.8% were married couples living together, 24.5% had a female householder with no husband present, 26.2% were non-families, and 22.4% of all households were made up of individuals. The average household size was 3.22 and the average family size was 3.82. The median age was 30.7 years. The median income for a household in the county was $31,335 and the median income for a family was $37,345. Males had a median income of $31,527 versus $26,236 for females. The per capita income for the county was $12,932. About 26.6% of families and 33.4% of the population were below the poverty line, including 43.0% of those under age 18 and 31.3% of those age 65 or over.
During its early history from 1912 to 1928, McKinley County voted for the Republican candidate in every presidential election. From 1932 onward, the county has backed Democratic candidates in all but three presidential elections that were national landslides for the Republican Party. This factor can be attributed to the county's large Native American majority which consists of over three-quarters of its population.
