Before reaching China, he taught at College General (Major Seminary) in Penang, Malaya from April 1821 to January 1822. In 1821, Bishop Esprit-Marie-Joseph Florens, the Vicar Apostolic of Siam, requested for him to call at Singapore. The bishop had been contemplating on opening a missionary station in Singapore. He was not very certain, though, whether there was any urgency nor was he aware of the circumstances prevailing in the island. Therefore, the young missionary was to go check on the situation.
He reached Singapore on 11 December 1821 and spent about a week there. Father Imbert could have been the first priest to celebrate Mass on the island. In 1822, he sailed for Macau, but unable to go directly there, he spent 2 years in Tonkin, French Indochina. In China, he spent twelve years in Szechuan province and founded a seminary in Moupin. On 26 April 1836, he was appointed Vicar Apostolic of Korea and Titular Bishop of Capsa. On 14 May 1837, he was ordained Titular Bishop of Capsa and crossed secretly from Manchuria to Korea the same year.
During this time, Korea was going through a period of Christian persecution. On 10 August 1839, Bishop Imbert, who was secretly going about his missionary work, was betrayed. Realizing that it was only a matter of time before he was arrested and killed, he celebrated Mass and surrendered himself to those who lay in waiting for him. He was taken to Seoul where he was tortured to reveal the whereabouts of foreign missionaries. Believing that his converts would be spared if all foreign missionaries came out from hiding and gave themselves up, he wrote a note to his fellow missionaries, Fathers Pierre-Philibert Maubant and Jacques-Honoré Chastan, asking them to surrender to the Korean authorities as well. He had written, “In desperate circumstances, the good shepherd lays down his life for his sheep.” They did and the three of them were imprisoned together. They were taken before an interrogator and questioned for three days to reveal the names and whereabouts of their converts.
As torture failed to break them down, they were sent to another prison and finally beheaded on 21 September 1839 at Saenamteo, Korea. Their bodies remained exposed for several days but were finally buried on Nogu Mountain. The three martyrs were one of the 79 Korean martyrs beatified in 1925 and were among the one hundred and three martyrs canonised by Pope John Paul II in Seoul on 6 May 1984.
