A log cabin and a steam-powered flour mill at Old Mill State Park are bold landmarks from the past. The grist mill still gets fired up once a year to grind flour. The Middle River meanders through the park and visitors can see deer, beaver, and moose. Enjoy the prairie and the ever-changing parade of wildflowers along wooded trails. A swimming area, swinging bridge, picnic grounds, campground, and interpretive displays are popular among visitors.

Old Mill State Park is like an island of original landscape in a sea of crop land. It contains examples of how the area must have looked to the early European settlers over a century ago. The riverine forest contains most of the same plant and animal species found here originally. Small areas of oak savanna and prairie can still be found on the beach ridges. Wet prairie still survives in the lower areas.

At one time, this entire northwest corner of the state was covered by a vast freshwater lake. Although the area was left fairly level by glacial activity before the lake formed, the lake is responsible for the large level areas found here. Over the centuries as the lake level dropped, large beach ridges were formed as new shoreline was exposed and carved out by the action of the waves. One of those ridges is located about one mile east of the park. The river valley was cut by the river flowing over the loose sediment left behind by the lake. Each time the lake level dropped, the river would cut a deeper channel. The steep-walled valley and narrow flood plain are typical of a young river valley.

The river valley that runs through the park acts as an access corridor for many species. Large mammals include deer and moose. Beaver and raccoon are common as are white-tailed jackrabbits and snowshoe hares. In spring and fall, birds migrating through this portion of Minnesota add another dimension to the park's wildlife. Warblers and finches, among others, are abundant during migration. You might even be able to observe magpies and scarlet tanagers. About 100 species of birds reside in the park. The ground nesting marsh hawk is a common summer resident. Broadwing and red-tailed hawks hunt small animals, and eagles are often spotted passing through. Short-eared and long-eared owls have been sighted in the park.

The historic site area in the park was originally homesteaded in 1882 by the Larson family. Beginning in 1886, a series of mills were built in and around the park. The original mill was water powered. When it was destroyed by floods, it was rebuilt in 1889 as a wind-powered mill. That mill, too, was destroyed and rebuilt a short time later as a water-powered mill in an area outside of the park. John Larson, son of the original homesteader, started a mill at the same location but powered it with a Case steam engine No. A359. In 1897, both mills were moved to the area in the park where the "old mill" now stands . The mills were owned and operated by several people before being sold to the state in 1937. Rebuilt in 1958, the steam engine and mill are fired up each year as part of park special events and interpretive programs. The beach ridges of the old lake served as travel routes for Indians, traders, and settlers. For the early settlers, the beach ridge just east of the park was one of the travel lanes. Known as the Pembina Trail, the oxcart trail and the river crossing are still visible.