Adjacent counties

• Williams County (north)
• Henry County (east)
• Putnam County (southeast)
• Paulding County (south)
• Allen County, Indiana (southwest)
• DeKalb County, Indiana (west)

2000 census

As of the census^[11] of 2000, there were 39,500 people, 15,138 households, and 11,020 families living in the county. The population density was 96 inhabitants per square mile (37/km²). There were 16,040 housing units at an average density of 39 units per square mile (15/km²). The racial makeup of the county was 92.59% White, 1.75% Black or African American, 0.26% Native American, 0.36% Asian, 0.02% Pacific Islander, 3.59% from other races, and 1.43% from two or more races. 7.23% of the population were Hispanic or Latino of any race. 45.6% were of German, 13.5% American, 6.8% Irish and 5.1% English ancestry according to Census 2000.

There were 15,138 households, out of which 34.30% had children under the age of 18 living with them, 58.90% were married couples living together, 9.60% had a female householder with no husband present, and 27.20% were non-families. 23.00% of all households were made up of individuals, and 9.50% had someone living alone who was 65 years of age or older. The average household size was 2.57 and the average family size was 3.02.

In the county, the population was spread out, with 26.50% under the age of 18, 9.20% from 18 to 24, 27.40% from 25 to 44, 23.90% from 45 to 64, and 12.90% who were 65 years of age or older. The median age was 36 years. For every 100 females there were 97.30 males. For every 100 females age 18 and over, there were 94.90 males.

The median income for a household in the county was $44,938, and the median income for a family was $50,876. Males had a median income of $37,936 versus $23,530 for females. The per capita income for the county was $19,667. About 4.50% of families and 5.60% of the population were below the poverty line, including 6.40% of those under age 18 and 5.30% of those age 65 or over.

2010 census

As of the 2010 United States Census, there were 39,037 people, 15,268 households, and 10,792 families living in the county.^[12] The population density was 94.9 inhabitants per square mile (36.6/km²). There were 16,729 housing units at an average density of 40.7 units per square mile (15.7 units/km²).^[13] The racial makeup of the county was 92.8% white, 1.9% black or African American, 0.3% Asian, 0.3% American Indian, 2.8% from other races, and 2.0% from two or more races. Those of Hispanic or Latino origin made up 8.7% of the population.^[12] In terms of ancestry, 42.3% were German, 12.5% were Irish, 9.6% were English, and 7.7% were American.^[14]

Of the 15,268 households, 32.4% had children under the age of 18 living with them, 55.1% were married couples living together, 10.8% had a female householder with no husband present, 29.3% were non-families, and 24.6% of all households were made up of individuals. The average household size was 2.51 and the average family size was 2.97. The median age was 39.4 years.^[12]

The median income for a household in the county was $44,480 and the median income for a family was $54,472. Males had a median income of $44,354 versus $30,610 for females. The per capita income for the county was $22,139. About 11.2% of families and 12.7% of the population were below the poverty line, including 18.0% of those under age 18 and 7.1% of those age 65 or over.^[15]

City

• Defiance (county seat)

Villages

• Hicksville
• Ney
• Sherwood

Townships

Unincorporated communities

• Evansport
• Farmer
• Jewell
• Mark Center