Edward and Camilla Hegeler commissioned W. W. Boyington to design the Mansion in 1874. Built in the Second Empire style, the Mansion is a grand presence in the Illinois Valley. The lavish 16,000-square-foot, seven-level home features a mansard roof; dormer windows; molded cornices; decorative brackets; and a tower crowned by a 30-foot cupola. The Mansion is a solid brick construction covered with a type of stucco that has been smoothed and tooled to resemble massive stone blocks. A horseshoe staircase leads guests to the main entrance of the home and to an elegant wrap-around porch that graces three sides of the residence, a full story above ground. Because zinc was readily available from the nearby Matthiessen & Hegeler Zinc Company, the metal is used throughout the Mansion. The flat roof, gutters, and downspouts are zinc, which does not rust.
The Mansion sits on part of what was once a three-acre estate that featured extensive walking paths, a gazebo, a reflecting pond, flower gardens, a greenhouse, tennis courts, and a railroad trolley car.