St. Clair County is located at the base of the 'thumb' in southeast Michigan, and shares an international border with Canada.  The population of the County in 2010 was 163,040 people, and it has 32 incorporated cities, townships and villages.  The total land area of the County is 725 square miles.

There are six major water bodies affecting St. Clair County. The largest of these water bodies is Lake Huron, which as part of the Great Lakes water system encompassing 12.5 miles of St. Clair County's boundaries. 

The St. Clair flats area is at the mouth of the St. Clair River. The flats represent one of the world's largest freshwater deltas in North America. Lake St. Clair, towards the southern end of the Great Lakes system, receives 218,000 cubic feet per second of water from the St. Clair River. Lake St. Clair supports a range of recreational activities and wildlife habitat.