Mackinac County is a county located in the Upper Peninsula of the U.S. state of Michigan. As of the 2010 census, the population was 11,113. The county seat is St. Ignace. Formerly known as Michilimackinac County, in 1818 it was organized as one of the first counties of the Michigan Territory, as it had long been a center of French and British colonial fur trading, a Catholic church and Protestant mission, and associated settlement.