Allegan County is primarily an agricultural area that is rapidly becoming urbanized as the population centers of Grand Rapids on the northeast and Kalamazoo to the southeast expand into Allegan County. A portion of the city of Holland lies within the northwest portion of the county, as does a larger urbanized area around it. In the southwest corner, a small portion of the city of South Haven extends into Allegan County. The Lake Michigan shoreline has long been a popular place for vacation homes, and that such development continues, especially around Saugatuck and Douglas.
The largest city wholly within the county is Allegan, the county seat.
The county has long been a regional tourist draw, particularly the Tulip Time Festival in Holland and the area along Lake Michigan. Another draw is Allegan State Game Area, a 45,000 acre (182 km²) forest attracting campers, snowmobilers, cross-country skiers and hunters. Allegan County also hosts Saugatuck Dunes State Park with trails through scenic dunes and a swimming beach on Lake Michigan.
As of the 2000 census, there were 105,665 people, 38,165 households, and 28,394 families residing in the county. The population density was 128 people per square mile (49/km²). There were 43,292 housing units at an average density of 52 per square mile (20/km²). The racial makeup of the county was 93.47% White, 1.31% Black or African American, 0.55% Native American, 0.55% Asian, 0.03% Pacific Islander, 2.77% from other races, and 1.32% from two or more races. 5.72% of the population were Hispanic or Latino of any race. 25.6% were of Dutch, 17.8% German, 9.6% American, 8.4% English and 7.2% Irish ancestry according to Census 2000. 93.6% spoke English and 5.2% Spanish as their first language.
There were 38,165 households out of which 37.40% had children under the age of 18 living with them, 61.40% were married couples living together, 9.10% had a female householder with no husband present, and 25.60% were non-families. 20.70% of all households were made up of individuals and 7.80% had someone living alone who was 65 years of age or older. The average household size was 2.72 and the average family size was 3.15.
In the county the population was spread out with 28.90% under the age of 18, 8.00% from 18 to 24, 30.00% from 25 to 44, 22.00% from 45 to 64, and 11.10% who were 65 years of age or older. The median age was 35 years. For every 100 females there were 99.60 males. For every 100 females age 18 and over, there were 97.60 males.
The median income for a household in the county was $45,813, and the median income for a family was $51,908. Males had a median income of $38,681 versus $26,887 for females. The per capita income for the county was $19,918. About 5.00% of families and 7.30% of the population were below the poverty line, including 7.50% of those under age 18 and 7.90% of those age 65 or over.