Monmouth County ranks 42nd among the highest-income counties in the United States. It also is ranked 53rd in the United States by personal per-capita income.
As of the census of 2000, there were 615,301 people, 224,236 households, and 160,328 families residing in the county. The population density was 1,304 people per square mile (503/km²). There were 240,884 housing units at an average density of 510 per square mile (197/km²). The racial makeup of the county was 84.39% White, 8.06% Black or African American, 0.14% Native American, 3.97% Asian, 0.02% Pacific Islander, 1.74% from other races, and 1.68% from two or more races. 6.20% of the population were Hispanic or Latino of any race. Based on the first ancestries reported by Monmouth County residents in the 2000 Census, 23% of residents were of Italian ancestry, 18% Irish, 9% German and 5% Polish ancestry.
There were 224,236 households out of which 35.50% had children under the age of 18 living with them, 58.20% were married couples living together, 10.00% had a female householder with no husband present, and 28.50% were non-families. 23.80% of all households were made up of individuals and 9.60% had someone living alone who was 65 years of age or older. The average household size was 2.70 and the average family size was 3.24.
In the county the population was spread out with 26.10% under the age of 18, 6.90% from 18 to 24, 30.40% from 25 to 44, 24.10% from 45 to 64, and 12.50% who were 65 years of age or older. The median age was 38 years. For every 100 females there were 94.40 males. For every 100 females age 18 and over, there were 90.90 males.
The median income for a household in the county was $64,271, and the median income for a family was $76,823. Males had a median income of $55,030 versus $35,415 for females. The per capita income for the county was $31,149. About 4.50% of families and 6.30% of the population were below the poverty line, including 7.50% of those under age 18 and 7.00% of those age 65 or over.
Monmouth County is home to a large Irish-American population, with 141,515 residents (23% of the county's population) listed themselves as being of Irish ancestry in the 2000 Census.