Daniel Kennedy opened a gristmill on Slippery Rock Creek in 1852. The mill was destroyed by fire in 1868 and was quickly rebuilt. Ownership of the mill was transferred to the park's namesake, Thomas McConnell in 1875. He replaced the waterwheel with water turbines and the grindstones with rolling mills. This made McConnell's Mill one of the first rolling mills in the country. The mill processed oats, corn, buckwheat and wheat until it was closed in 1928. Ownership of the land transferred from Thomas H. Hartman to the Western Pennsylvania Conservancy in 1942. The conservancy then transferred the land to the Commonwealth of Pennsylvania in 1957 when McConnells Mill State Park was formally dedicated.