As of the census of 2000, there were 556,678 people, 220,936 households, and 141,178 families residing in the county, making Bernalillo the most populated county in the state. The population density was 477 people per square mile (184/km²). There were 239,074 housing units at an average density of 205 per square mile (79/km²). The racial makeup of the county was 70.75% White, 2.77% Black or African American, 4.16% Native American, 1.93% Asian, 0.10% Pacific Islander, 16.07% from other races, and 4.22% from two or more races. 41.96% of the population were Hispanic or Latino of any race.
There were 220,936 households out of which 31.40% had children under the age of 18 living with them, 46.00% were married couples living together, 12.90% had a female householder with no husband present, and 36.10% were non-families. 28.50% of all households were made up of individuals and 7.90% had someone living alone who was 65 years of age or older. The average household size was 2.47 and the average family size was 3.06.
In the county the population was spread out with 25.30% under the age of 18, 10.30% from 18 to 24, 30.40% from 25 to 44, 22.40% from 45 to 64, and 11.50% who were 65 years of age or older. The median age was 35 years. For every 100 females there were 95.50 males. For every 100 females age 18 and over, there were 92.90 males.
The median income for a household in the county was $38,788, and the median income for a family was $46,613. Males had a median income of $33,720 versus $26,318 for females. The per capita income for the county was $20,790. About 10.20% of families and 13.70% of the population were below the poverty line, including 17.90% of those under age 18 and 9.10% of those age 65 or over.