He was parish priest in Acadia from about 1692 until 1698 during the period of Joseph Robineau de Villebon and appears to have been less than successful in this pursuit.
Despite his lack of success in Acadia, Father St-Cosme become one of the pioneers of the missionary work by the Jesuits in the Mississippi Valley.
Sometime around 1705 Tattooed Arm gave birth to St. Cosme's son who would be the last Great Sun before the French ultimately tried to eradicate the Natchez in retribution for the massacres of Frenchmen, women and children in 1729.
Saint Cosme's son, the Natchez King, was sent to the West Indians in 1731 and made to work as a slave.
When encamped on the banks of the Mississippi River, they were murdered and robbed by Chitimacha men.