Since the castle was built by the St. John family in around 1200, it has only changed hands once. Oliver St John of Fonmon was one of the Twelve Knights of Glamorgan who effected the Norman conquest of Glamorgan. The St John family is today represented by the Viscounts Bolingbroke. Fonmon is still a private residence and the present owners, the Boothby baronets, are descendants of Colonel Philip Jones, who bought the house in 1656. Notable features include the library, designed by Thomas Stocking and the extensive gardens.
The castle is open to the public from April to September; access to the garden and grounds is free. It is also a wedding and events venue, hosting the Vale of Glamorgan Agricultural Show and the St Davids Polo.