what county is algonac in?
Homefacts City Report. Algonac is located in St. Clair County, MI. The population is 4,127, making Algonac the 5th largest city in St. Clair County. There are 2 public schools in Algonac with an average Homefacts rating of B+. The total crime rate for Algonac is moderate, and there are 13 registered sex offenders residing in the city.