St. Ignace is a city in the U.S. state of Michigan and the county seat of Mackinac County. The city had a population of 2,306 at the 2020 cen…St. Ignace is a city in the U.S. state of Michigan and the county seat of Mackinac County. The city had a population of 2,306 at the 2020 census. St. Ignace Township is located just to the north of the city; the two are administered separately.