Built 167 years ago in 1858, it is a rectangular, one-story, gable-front, frame structure in the Gothic Revival style.

St. Mary's was the first Catholic church built within Fairfax County, and its early parishioners were primarily Irish immigrants employed by the Orange and Alexandria Railroad.

During the Civil War, wounded were brought here by train to be treated and evacuated to Alexandria and Washington after the Second Battle of Bull Run (Manassas) in late August 1862.

Volunteer Clara Barton, an employee of the U.S. Patent Office in Washington, tended to the wounded and made this church her headquarters;[3] she later founded the American Red Cross in 1881.

The annual Labor Day picnic in early September continues to be held on the grounds (primarily its graveyard),[5] and is one of the oldest celebrations in the county.