Carlinville is a city in and the county seat of Macoupin County, Illinois, United States. As of the 2020 census, the population was 5,710. It is …Carlinville is a city in and the county seat of Macoupin County, Illinois, United States. As of the 2020 census, the population was 5,710. It is the home of Blackburn College, a small college affiliated with the Presbyterian church. The city is the former home of Prairie Farms Dairy.