Originally part of McHenry County, the townships comprising Lake County were carved out into a separate county in 1839.
As of the census of 2000, there were 644,356 people, 216,297 households, and 163,953 families residing in the county. The population density was 1,440 people per square mile (556/km²). There were 225,919 housing units at an average density of 505 per square mile (195/km²). The racial makeup of the county was 80.11% White, 6.94% Black or African American, 0.28% Native American, 3.90% Asian, 0.05% Pacific Islander, 6.72% from other races, and 2.01% from two or more races. 14.39% of the population were Hispanic or Latino of any race. 17.0% were of German, 9.0% Irish, 7.2% Polish, 6.4% Italian and 5.0% American ancestry according to Census 2000. 78.7% spoke English, 12.9% Spanish and 1.0% Russian as their first language.
The Census estimates of 2005 showed Lake County with 68.6% of its residents being non-Hispanic whites, 18.2% Latinos, 6.8% African American and 5.5% Asian.
According to the 2000 census there were 216,297 households out of which 42.10% had children under the age of 18 living with them, 63.20% were married couples living together, 9.20% had a female householder with no husband present, and 24.20% were non-families. 19.70% of all households were made up of individuals and 6.20% had someone living alone who was 65 years of age or older. The average household size was 2.88 and the average family size was 3.33.
In the county the population was spread out with 29.40% under the age of 18, 8.90% from 18 to 24, 31.60% from 25 to 44, 21.60% from 45 to 64, and 8.50% who were 65 years of age or older. The median age was 34 years. For every 100 females there were 101.20 males. For every 100 females age 18 and over, there were 99.30 males. Waukegan and North Chicago account for 20% of the county's population.
The median income for a household in the county was $66,973, and the median income for a family was $76,424 (these figures had risen to $76,940 and $88,851 respectively as of a 2007 estimate). Males had a median income of $50,789 versus $33,458 for females. The per capita income for the county was $32,102. About 4.00% of families and 5.70% of the population were below the poverty line, including 7.00% of those under age 18 and 4.80% of those age 65 or over. The 2000 Census reported that Lake County had the second median highest household income of any county in the state, behind DuPage County.