St. Clair County is a county located in the U.S. state of Michigan bordering the St. Clair River. As of the 2010 census, the population was 163,040. It is the 13th-most populous county in the state, and the county seat is Port Huron, located at the north end of the St. Clair River at Lake Huron. The county was created September 10, 1820, and its government was organized in 1821.