The Gatien Bonnet museum carries the name of its founder, the town's mayor, who opened it to the public in 1868.
Reorganised in 1955 in the Town Hall, in 1962 the collections were moved to the building that still houses them, on two levels, by the Square Foucher de Careil, close to the banks of the Marne.
Devoted to the history of Lagny and its region, from prehistory to the beginning of the 20th century, the museum presents particularly neolithic tools, medieval ceramics and religious art. Work by Lagny’s neo-impressionist group (Cavallo-Peduzzi, Paul-Emile Colin, Léo Gausson, Emile Prodhon) has a place of honour. The collection also includes works and manuscripts by the writer Léon Bloy.