The Thomas House is a two-story brick manor house constructed ca. 1780 by James Marshall. The house and its surrounding 240-acre farm was purchased in 1860 by C. K. Thomas, a retired merchant. The house sustained major damage during the Battle of Monocacy, and remained in the Thomas family until the early 20th century.
The National Park Service acquired the Thomas Farm in 2001. Soon after acquisition of the house, it became apparent that the slate roof covering had reached the end of its lifecycle and needed to be replaced. It was also clear that the three dormer windows needed significant repair and rehabilitation in order to preserve them and to ensure their continued function. The slate roof replacement and dormer window project began in fall 2006, and was completed in spring 2007.