He governed the island of Saint Christopher from 1639 to his death in 1660, first under the Compagnie des Îles de l'Amérique and later under the Knights of Malta themselves.
On 12 January 1638 Poincy set sail for the Caribbean on board La Petite Europe On February 20 he took up his commission as Lieutenant Governor of the Isles of America and Captain general of the French at St Kitts.
On 26 December 1644, the French king sent Noël Patrocle de Thoisy to relieve him, Poincy refused to allow him to land.
Then he sent his nephew, Robert de Lonvilliers, with 300 men to take over the French half of Saint Martin.
In 1650 he heard that the Spanish had evacuated St Croix, so he sent Vaugelan with two ships and one hundred and sixty men to capture it.
Following the intervention of the Knights of Malta in 1651, Poincy paid 90,000 livres to make peace with Thoissy.
He was a Bailiff Grand cross of the Knights of Malta and Squadron commander of the French Brittany Fleet.