In 1856, de Montigny was sent as a French envoy to King Mongkut of Thailand.
[2] A treaty was signed on 15 August 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.
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 Charles Rigault de Genouilly on 1 September 1858.
[2] Charles de Montigny served again as President of the Municipal Council in Shanghai from 1 May 1862.