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. Flat Rock is home to the AutoAlliance International manufacturing plant, co-owned by Ford Motor Company and Mazda Motor Corporation. As of 2005 it produces the Mazda 6 and the 5th generation Ford Mustang. Flat Rock is the namesake of the famous Flat Rock Speedway, located in Berlin Charter Township in neighboring Monroe County.