The racial makeup of the county was 88.70% White, 4.56% Black or African American, 0.21% Native American, 0.92% Asian, 0.02% Pacific Islander, 3.06% from other races, and 2.52% from two or more races. 2.44% of the population were Hispanic or Latino of any race.

There were 168,361 households out of which 36.30% had children under the age of 18 living with them, 57.00% were married couples living together, 11.90% had a female householder with no husband present, and 27.30% were non-families. 22.20% of all households were made up of individuals and 9.00% had someone living alone who was 65 years of age or older. The average household size was 2.74 and the average family size was 3.23.

In the county the population was spread out with 26.80% under the age of 18, 7.20% from 18 to 24, 30.40% from 25 to 44, 23.90% from 45 to 64, and 11.80% who were 65 years of age or older. The median age was 37 years. For every 100 females there were 95.00 males. For every 100 females age 18 and over, there were 91.30 males.

The median income for a household in the county was $55,615, and the median income for a family was $65,554. Males had a median income of $45,535 versus $31,389 for females. The per capita income for the county was $24,789. About 4.90% of families and 6.60% of the population were below the poverty line, including 8.30% of those under age 18 and 7.90% of those age 65 or over.

The leading ancestry group in Plymouth County is Irish, with 31%. Plymouth County, along with Norfolk County, Massachusetts, claims the highest percentage of people with Irish ancestry in the United States.