Willacy County is part of the Raymondville Micropolitan Statistical Area as well as the Brownsville–Harlingen–Raymondville Combined Statistical Area.
As of the census of 2000, there were 20,082 people, 5,584 households, and 4,584 families residing in the county. The population density was 34 people per square mile (13/km²). There were 6,727 housing units at an average density of 11 per square mile (4/km²). The racial makeup of the county was 70.37% White, 2.19% Black or African American, 0.50% Native American, 0.11% Asian, 0.03% Pacific Islander, 24.46% from other races, and 2.34% from two or more races. 85.69% of the population were Hispanic or Latino of any race.
There were 5,584 households out of which 42.90% had children under the age of 18 living with them, 62.10% were married couples living together, 16.10% had a female householder with no husband present, and 17.90% were non-families. 16.50% of all households were made up of individuals and 9.70% had someone living alone who was 65 years of age or older. The average household size was 3.40 and the average family size was 3.85.
In the county, the population was spread out with 31.60% under the age of 18, 11.90% from 18 to 24, 26.60% from 25 to 44, 18.30% from 45 to 64, and 11.60% who were 65 years of age or older. The median age was 30 years. For every 100 females there were 105.20 males. For every 100 females age 18 and over, there were 104.90 males.
The median income for a household in the county was $22,114, and the median income for a family was $25,076. Males had a median income of $19,706 versus $15,514 for females. The per capita income for the county was $9,421. About 29.20% of families and 33.20% of the population were below the poverty line, including 42.00% of those under age 18 and 29.90% of those age 65 or over. The county's per-capita income makes it one of the poorest counties in the United States.