Trnovo Mansion, named after the eponymous village, which now forms part of Ilirska Bistrica, is one of those early modern mansions and seigniorial seats, the location of which has sunken into oblivion and the very existence of which has remained forgotten for a very long time. There are two main reasons for this. First, the small seigniory, named Trnovo Mansion, only existed for a part of the eighteenth century, and the appearance of the building itself was in no regard so distinct as to be later necessarily associated with the former seigniorial seat. Founded by the Barons Oberburg, the seigniory de facto ceased to exist as a legal entity in 1762, when its estate was divided among Baron Wolf Anton’s five heirs. At that time, the mansion itself, no more than a fine house, belonged to the baron’s sister, who, two decades later, handed it over to her relative from another branch of the Oberburg family. Once it passed into non-noble hands, the memory of its former function was definitively lost. The article methodologically presents the process of tracing the construction, location, and disappearance of the mansion.