The historic Thomas House is a stately 18th-century brick manor house. It was likely constructed by a Scottish merchant named James Marshall and was occupied by Christian Keefer Thomas during the Battle of Monocacy. Although it has been signficantly altered over the past two centuries, it is in remarkably good condition and features a number of important 18th- and 19th-century architectural details.
The Thomas Farm and its associated historic structures were purchased in 2001, but the main house remained subject to a life tenancy. In early 2008, Monocacy National Battlefield assumed possession of the Thomas House.