Museum as a legal organization was established about fifty years after the last owners of the estate had left it. Nothing remained  by 1980s from historical design, decorations and art collections of the mansion.

Mainly two owners, Ludwig Nickolay (1737-1820) and his son, Paul Nickolay (1777-1866) emphasized their efforts in forming  collections. For many years Ludwig Nickolay  collected books  and masks and casts of the ancient gemmas. In his library there were a lot of  ancient books in art history. His son significantly refilled and methodized it by making himself a handwritten catalog. The book collection that was donated  to Helsinki University by the owners in  XX c.,  consisted of about  5000 editions of more than 9000 volumes. It is one of the biggest private book collections in Finland, which actually started from the library of two friends: Ludwig Nicolay and Fransua Lafermier, his university mate and colleague in Russian diplomatic service. 

Later Paul Nickilay started to collect paintings. According to the first handwritten catalogue of 1823, the collection was composed of 86 works by contemporary French painters  Lantaur, Bouasie, Charden and Dutch painter Rujsdall  of XVII c . During Winter War of 1939-1940, a lot of items of the collection were evacuated by the owners.  Some works of paintings and sculptures that were left came to the funds of Hermitage state museum.

The Monrepos museum collection started only in 1990s,  from the archeological stock found during archeological expeditions. The oldest item of the archeological fund is the part of the Swedish armor dated  XVII.

Within the book collection (currently 700 volumes)  there are the analogues of the famous “Hydraulic  architecture”  by Belidour (Paris, 1737-1753), "History of Emperor Carl V" by Roberson  (London, 1769), "History of ancients’ art" Winkelmann Paris, 1802-1803), antique books on art, culture and history of Russia and Finland, contemporary publications on Monrepos’ history and owners and other  documental materials.