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.

Thomas House 1st Floor Plan

Long-term planning calls for adaptive reuse of the Thomas House as the park's administrative headquarters, which will require significant improvements in its HVAC, electrical, telephone, plumbing, and other systems. Prior to undertaking any signficant interior alterations, Monocacy National Battlefield has initiated a historic structure report (HSR) as well as an adaptive re-use plan for Thomas House. These studies require extensive historical and architectural research and documentation, and will help trace the evolution of the house over time and also identify its character-defining features. These studies also provide several alternatives for upgrading the house for administrative use.

These planning efforts will ensure that any changes or improvements to the house will not have an adverse impact on its architectural integrity, but also ensure that the building will be safe and accessible for staff occupancy.

Projects at the Thomas House are being performed by the Monocacy National Battlefield Cultural Resources Division and the National Park Service's Historic Preservation Training Center (HPTC). HPTC is headquartered at Monocacy National Battlefield in the historic Gambrill House.