One of the most interesting examples of sepulchral sculpture in mid-19th century European art, combining historicist references and realistic characterisation of the figures. It is currently located in a niche in the side aisle, in a chapel with neo-Gothic forms: two recumbent figures, above them a standing angel figure. On the plaque in the niche, the merits of the deceased are inscribed in Polish and Latin. Next to it the coat of arms of the Republic.

The monument to Niemcewicz was commissioned from Oleszczyński by the Historical and Literary Society in Paris on 3.07.1841. It was a heroic bust of the writer. However, after it was made, due to Kniaziewicz's death in May 1842, the concept of the tomb was changed: it was the general's will to rest next to his friend, Niemcewicz. Thus, in June 1842, Oleszczyński was already planning a double tomb and made a local visit to Montmorency with Jan Koźmian. By the end of 1843, Oleszczyński had drawn up three different designs for the monument (including one with standing figures, which he described as 'heroic'). The third design, called 'religious-monumental' by the artist, was chosen to be realised in marble, with recumbent figures and a statue of an angel standing above them, harmonising well with the late Gothic architecture of the church. The deceased are depicted in contemporary attire; their posing, however, indicates a desire to refer to Gothic tombstones, in line with the trend then visible in both Polish (Jakub Tatarkiewicz's tomb of the Potocki family in Wilanów) and French sculpture. In 1867, in his memoir on the late artist, J.I. Kraszewski wrote that the tomb in Montmorency "is a simple and beautiful idea. Two friends rest as if in a dream embraced side by side, while an angel of resurrection watches over their heads'. The figures of the deceased, although portrayed with great resemblance and without idealisation, are inscribed in the symbolic concept of the monument, which is at the same time intended to depict the history and personal merits of the two protagonists (as emphasised by the inscriptions on the tombstone). The allegorical idea is tied together by the figure of an angel who "(...) stopped over his deceased friends, and with a miraculous movement of his blessing hands, which he spread sweetly over the grey heads of the soldier and the poet, let it be known that he wants to watch over their sleepless sleep (...)" (F. Hoesick).

The monument, which was awarded a medal at the Paris Salon of 1848 (there it bore the title "The Angel of Peace watches over Niemcewicz and Kniaziewicz"), was originally placed in a richly decorated chapel, added in 1850 especially for the "Mausoleum of Poles" (as the French christened it). Since the beginning of the 20th century, the monument has been located in a small neo-Gothic niche at the close of the side nave of the church, symmetrically placed in relation to the also neo-Gothic epitaph of Father Adam Jerzy Czartoryski. It is a cenotaph; the actual tomb of both politicians is in the Champeaux cemetery in Montmorency.

The collegiate church of Montmorency, once filled with the magnificent Renaissance tombs of local princes (destroyed during the Revolution), then became a sanctuary for Polish émigrés. In addition to a monument to Kniaziewicz and Niemcewicz, there are many commemorative plaques here. Before returning to Poland in 1857, Oleszczyński made several more tombstones in Paris cemeteries, as well as in Auxerre and Chantilly.