St. Clair is a city in St. Clair County in the eastern "Thumb" of the U.S. state of Michigan. The population was 5,485 at the 2010 census. Th…St. Clair is a city in St. Clair County in the eastern "Thumb" of the U.S. state of Michigan. The population was 5,485 at the 2010 census. The city is located on the St. Clair River near the southeast corner of St. Clair Township.