As is the case with all eight of Connecticut's counties, there is no county government, and no county seat. In Connecticut, towns are responsible for all local government activities, including fire and rescue, snow removal and schools. In a few cases, neighboring towns will share certain resources, i.e. water, gas, etc. However, Middlesex County is merely a group of towns on a map, and has no particular authority.
As of the census of 2000, there were 155,071 people, 61,341 households, and 40,607 families residing in the county. The population density was 420 people per square mile (162/km²). There were 67,285 housing units at an average density of 182 per square mile (70/km²). The racial makeup of the county was 91.28% White, 4.42% Black or African American, 0.17% Native American, 1.56% Asian, 0.04% Pacific Islander, 0.97% from other races, and 1.56% from two or more races. 3.00% of the population were Hispanic or Latino of any race. 20.4% were of Italian, 14.3% Irish, 11.1% English, 8.9% Polish and 8.2% German ancestry according to Census 2000. 91.1% spoke English, 2.6% Spanish, 1.9% Italian, 1.2% French and 1.0% Polish as their first language.
There were 61,341 households out of which 30.30% had children under the age of 18 living with them, 54.40% were married couples living together, 8.80% had a female householder with no husband present, and 33.80% were non-families. 27.20% of all households were made up of individuals and 10.00% had someone living alone who was 65 years of age or older. The average household size was 2.43 and the average family size was 2.98.
In the county the population was spread out with 23.20% under the age of 18, 7.30% from 18 to 24, 31.10% from 25 to 44, 24.80% from 45 to 64, and 13.60% who were 65 years of age or older. The median age was 39 years. For every 100 females there were 95.10 males. For every 100 females age 18 and over, there were 92.10 males.
The median income for a household in the county was $59,175, and the median income for a family was $71,319. Males had a median income of $48,341 versus $35,607 for females. The per capita income for the county was $28,251. About 2.30% of families and 4.60% of the population were below the poverty line, including 4.00% of those under age 18 and 5.90% of those age 65 or over.