Hidalgo was created on February 25, 1919 from the southern part of Grant County. According to the 2001/2 New Mexico Blue Book, it was named for the town north of Mexico City where the Treaty of Guadalupe Hidalgo was signed. Other sources cite Miguel Hidalgo as the county's namesake.
As of the census of 2000, there are 5932 people in the county, organized into 2152 households, and 1542 families. The population density is 2 people per square mile (1/km²). There are 2,848 housing units at an average density of 1 per square mile (0/km²). The racial makeup of the county is 83.78% White, 0.78% Native American, 0.40% Black or African American, 0.32% Asian, 0.00% Pacific Islander, 11.85% from other races, and 2.87% from two or more races. About 56% of the population is Hispanic or Latino of any race.
There are 2,152 households out of which 37.50% have children under the age of 18 living with them, 53.80% are married couples living together, 13.60% have a female householder with no husband present, and 28.30% are non-families. 25.30% of all households are made up of individuals and 10.70% have someone living alone who is 65 years of age or older. The average household size is 2.72 and the average family size is 3.29.
In the county the population is spread out with 31.70% under the age of 18, 7.80% from 18 to 24, 25.20% from 25 to 44, 21.70% from 45 to 64, and 13.60% who are 65 years of age or older. The median age is 35 years. For every 100 females there are 99.60 males. For every 100 females age 18 and over, there are 97.00 males.
The median income for a household in the county is $24,819, and the median income for a family is $31,552. Males have a median income of $25,960 versus $19,531 for females. The per capita income for the county is $12,431. 27.30% of the population and 23.90% of families are below the poverty line. Out of the total population, 38.90% of those under the age of 18 and 17.00% of those 65 and older are living below the poverty line.