He stayed in London as chargé d'affaires as when Bellièvre left in late January 1640, returning to France himself in early summer 1641 when the Marquis de La Ferté-Imbault was appointed ambassador.
[1] For just over two years from February 1642 until the spring of 1644 Montereul was secretary to the Marquis de Fontenay-Mareuil, the French ambassador in Rome.
In June Montereul returned to France with Charles holding out false hopes of French diplomatic aid.
[1] When Charles was transferred to English custody and lodged in Holmby House, Montereul left him and travelled to Edinburgh.
Since Montereul's first arrival in England a key concern of Mazarin had been the importance of raising troops for French service.
Remaining faithful to the prince of Conti and to the duke of Longueville, he entered into a secret correspondence with them during their imprisonment in 1650, but died of tuberculosis aged 36 or 37, shortly after they were freed.