As of the census of 2000, there were 142,361 people, 39,384 households, and 31,467 families residing in the county. The population density was 34 people per square mile (13/km²). There were 43,891 housing units at an average density of 10 per square mile (4/km²). The racial makeup of the county was 49.37% White, 3.95% Black or African American, 1.87% Native American, 1.99% Asian, 0.08% Pacific Islander, 39.08% from other races, and 3.65% from two or more races. 72.22% of the population were Hispanic or Latino of any race. 65.7% spoke Spanish and 32.3% English as their first language.
By 2005 the estimated percentage of non-Hispanic whites in the county was 18%. The percentage of Hispanics had risen to 75.3. The African-American percentage now stood at 4.2%, showing that this population was growing significantly in the county. The number of Hispanics rose about 10 times since the first US census in 1910, but there are third or fourth-generation Mexican Americans whom identify more with Anglo-American culture but preserved their Hispanic heritage, such as usage of the Spanish language.
Imperial County has a long history of Asian Americans, although small in number compared to urban centers in California. Mainly of Chinese, but also of Filipino, Japanese, Korean and a scant number of Southeast Asian ancestry, often are descendants of railroad workers, ditch diggers and farm laborers in the early 20th century, some arrived through the Mexican border. Imperial County is also home to 10,000 Indian Americans, the highest number outside a US major city, and they provided some Indian features in a mainly Latin American culture.
There were 39,384 households out of which 46.7% had children under the age of 18 living with them, 57.7% were married couples living together, 17.1% had a female householder with no husband present, and 20.1% were non-families. 17.1% of all households were made up of individuals and 8.1% had someone living alone who was 65 years of age or older. The average household size was 3.33 and the average family size was 3.77.
In the county the population was spread out with 31.4% under the age of 18, 9.9% from 18 to 24, 30.4% from 25 to 44, 18.2% from 45 to 64, and 10.0% who were 65 years of age or older. The median age was 31 years. For every 100 females there were 109.3 males. For every 100 females age 18 and over, there were 111.4 males.
The median income for a household in the county was $31,870, and the median income for a family was $35,226. Males had a median income of $32,775 versus $23,974 for females. The per capita income for the county was $13,239. About 19.4% of families and 22.6% of the population were below the poverty line, including 28.7% of those under age 18 and 13.6% of those age 65 or over.
By 2006 the population had risen to 160,201, the population growth rate since the year 2000 was 30% the highest in California and 5th highest in the US in the time period. High levels of immigration, new residents search for affordable homes, and some came for retirement is to explain such a population increase.
Imperial County is served by Greyhound Lines and Imperial Valley Transit buses. Amtrak trains also travel through the county, but with no scheduled stops.