Flat Rock is a city in Wayne County, Michigan, United States, with a small portion of the city extending into Monroe County. At the 2020 census…Flat Rock is a city in Wayne County, Michigan, United States, with a small portion of the city extending into Monroe County. At the 2020 census, the population was 10,541. Flat Rock began as a Wyandot settlement. It was later designated as a reservation for the Wyandot, and still functioned as such in 1830.