In 1856, de Montigny was sent as a French envoy to King Mongkut of Thailand. A treaty was signed on August 15, 1856 to facilitate trade, guarantee religious freedom, and allow the access of French warships to Bangkok.
From Thailand, de Montigny visited Vietnam in 1857 to demand the establishment of a consulate in Huế, freedom to trade and to preach, and an end to persecution against Catholics. However, the Vietnamese court rejected all of his demands. When Montigny's mission failed, Napoléon III decided to dispatch a military force of 3,000 to Vietnam, leading to the capture of Da Nang by Rigault de Genouilly on September 1, 1858.
Charles de Montigny served again as President of the Municipal Council in Shanghai from May 1, 1862. From 1863 to 1868, he was French Consul in Tientsin, where he died in 1868.