Flat Rock is a city in Wayne County of the U.S. state of Michigan. A very small portion of the city extends into Monroe County. At the 2010 census, the city population was 9,878. Flat Rock is home to the Flat Rock Assembly Plant, owned by Ford Motor Company. As of 2012 it produces the 5th generation Ford Mustang, and is soon to make the Ford Fusion. Flat Rock is the namesake of the famous Flat Rock Speedway, located in Berlin Charter Township in neighboring Monroe County.