Around the year 1741, Abraham Barnes arrived in St. Mary's County and started buying up parcels of land. By 1742, he had acquired over 1000 acres in the Leonardtown area and had obtained a resurvey and grant of these acres under the title America Felix Secundus. The original dwelling house, started sometime after 1742, was probably a small house and typical of the times, a wide central hall with a room on either side and a second floor of dormer bedrooms. Renovations incurred as early as the 1760s were reflected in the tax rolls of 1798 which describe the house as "being of wood with one story brick wings, a kitchen, a store, a meat house and a corn house in good repairs". The house remained in possession of the Barnes heirs until about 1817 when it was sold to Phillip Key.
Phillip Key raised the roof and squared the house out into the 14-room mansion we know today. From 1830 to 1949, the house was passed down from fathers to sons. The Phillip Key family was large and well known. One relative, Francis Scott Key, wrote "The Star Spangled Banner". It is felt by many that the name Tudor Hall was given to the house/estate by a 19th century owner, Henry G. S. Key. In 1949, the Key family placed the property on the Market.
The house was purchased in 1949 by Mary Patterson Davidson, and her plan was to restore it and give it to the county as a library. Mrs. Davidson's untimely death did not stop her ideas from becoming reality as friends took over and saw that her plans were accomplished. By 1967-68, repairs were needed, and an effort was undertaken to restore the house to its 1830 configuration. Tudor Hall continued as the St. Mary's County Memorial Library until February 1984, when the library was moved because the structure could not accommodate room for the library's growth.
The St. Mary's County Historical Society purchased Tudor Hall on December 31, 1984 and the second floor is used as the Society's Research Library.
The unusual features of the house are the inset portico and beautiful hanging staircase in the main hall. This wide hall led through the house and down into the garden with a view of the Yew Tree, where members of the Barnes family are known to have been buried. The curving stairway led up to the second floor - with its eight bedrooms, six with fireplaces. It is known that George Washington visited Tudor Hall and most probably "slept here". Other features include the kitchen - with its triple fireplace, a much-used curving back stairs, the captain's walk on the roof, and the "haw-haw" fence.