Mermet-Cachon returned to Edo and served as an interpreter for Prince Gustave Duchesne de Bellecourt, who was a strong advocate of the use of force to govern relations with Japan and who supported the French intervention in the 20 July 1863 Bombardment of Shimonoseki. After returning to France in July 1863, Mermet-Cachon abandoned the Paris Foreign Missions Society, and after renouncing his vows as a Catholic priest, returned to Japan in a private capacity as interpreter to the French diplomat Léon Roches in April 1864. On 1 April 1865, he established a French language school in Yokohama and took a former prostitute as his common-law wife. He returned to France again on 27 October 1866, apparently with the intention of making only a short stay; however, in 1867, he was requested to serve as an assistant to Tokugawa Akitake, the 14-year-old son of Tokugawa Nariaki, "daimyō" of Mito Domain, who had arrived as a special envoy of the Tokugawa shogunate for the 1867 World Fair in Paris, where Japan had a pavilion. He also acted as interpreter for Tokugawa Akitake's audience with Napoleon III, which subsequently resulted in strong French support for the Tokugawa shogunate. Mermet-Cachon never returned to Japan, but died in Cannes on 14 March 1889. His grave is at the Père Lachaise Cemetery in Paris.