Willowbrook is made up of more than 100 feet of deposits left behind when the Wisconsin Glacier melted. Those deposits, part of the Valparaiso Moraine, give the preserve its contours and contain a mix of rock from the bottom Lake Michigan, chunks of Canadian Shield granite and samples from all points in between. Ancient stone spear points found at the preserve indicate that early hunters traversed the land in search of game.

An land survey from the 1830s indicated the preserve was prairie and used primarily for agricultural.

In the early 1900s Al and Audrie Chase bought the land for a weekend and summer retreat from their work at the Chicago Tribune. In 1956 Audrie Chase donated 45 acres to the Forest Preserve District, the first donation of its kind in District history. The Chases asked that the land be named Willow Brook, a nod to the name they had given the creek that flows through the property, and that a portion remain a sanctuary for birds.