There is a Benedictine monastery with the Church of Saint Mary on the Isle of Saint Mary situated in the southern part of the Great Lake. The area of the isle is 1.2 acres (0.5 hectares). The monastery was being built from 1177 to 1198. Initially, the Benedictines built the monastery and later on the church. During centuries, the church changed its appearance under the influence of different styles, the Renaissance and Baroque, but the central part remained Romanesque. The Church of Saint Mary as part of the monastery compound is a single nave Romanesque (Apulian) construction, built after the original Benedictine’s building in Monte Gargano. The church was partitioned and extended in the 16th century when the coat of arms of Gundulić family was installed on the porch. On several occasions, alterations were made in the Romanesque monastery. At present time, the building is a two-floor Renaissance building enclosed with courtyards on two sides and having arcaded main tract facing the courtyards. A defensive tower was built in the south-eastern corner therefore all the buildings, including the church, became defensive structures. The monastery was also reconstructed during the Renaissance, thus creating a uniform complex with the church.
History of the monastery extends as far as 1198 when the Pope Innocent III issued a document consecrating the Church of Saint Mary on the isle of the same name in the Great Lake on the island of Mljet. The Benedictines organised the monastery in accordance with the Rule of Saint Benedict. For a long time, the Benedictines were taking a good care of the island and lived in harmony with nature praising God. Many significant names, like Mavro Vetranović and Ignjat Đurđević, could be found among the Benedictines of the monastery. In 1345, the Benedictines renounced their rule over a part of the island therefore Mljet obtained a Statute and municipality in Babino Polje. Formally, it was annexed by the Republic of Ragusa in 1410. The monastery continued with its activities until 1809, when it was closed during the rule of Napoleon. From that time on, the island had a number of different owners; the monastery property was governed by the state, and the monastery became more and more neglected. In 1960, the monastery was converted into a hotel and was opened until 1991. In 1998, the dilapidated monastery was given back to the Diocese of Dubrovnik. Renovation and redecoration works are currently ongoing.