On 12 January de 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. He arrived wearing the regalia of the Knights of St John and soon dispensed with the authority of the French king, declaring "The people of St Kitts will have no other Governor than De Poincy and will take no orders from the King of France."
In 1639 he reached an agreement with the English on St Kitts that neither nation should grow tobacco. for one and a half years.
He instructed one of his followers, the Huguenot Levasseur with sixty buccaneers to drive out the English from Tortuga. Levasseur was successful and on 6 November 1640 a treaty was drawn up between de Poincy and Levasseur which allowed religious tolerations and trade between the two islands.
By 1642 he started building the Chateau de Montagne on his estate called La Fontaine. This was an elaborate building, credited as being one of the grandest ever constructed in the Americas, though today it is in ruins. The grounds of La Fontaine were also heavily planted with exotic tropical plants. He specifically imported the flamboyant plant from Madagascar for his garden, and is credited with introducing the beautiful plant to the Americas. It was thus re-named Royal Poinciana in his honour.
He had a Town Hall erected in Basseterre in what is now known as Church Street. This served as his administrative centre, where he dispensed justice and administered the colony.
On 26 December 1644, the French king sent de Thoissy to relieve him, de Poincy refused to let him land. Eventually de Thoissy was sent back to France in chains. The Capuchins were also expelled at this time for taking the side of de Thoissy. The Jesuits were invited in to take their place. De Poincy bought the nearby island of St Croix, which he bequeathed to the Knights of St John.
In 1648 first he seized the island of St Bartholomew, populated by 170 Europeans and fifty enslaved Africans. Then he sent his nephew, Robert de Lonvillliers, with 300 men to take over the French half of Saint Martin. This was ratified at the Treaty of Concordia. 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. The French set fire to the trees which had made settling there so problematic for the Spanish.
Following the intervention of the Knights of Malta in 1651 he paid 90,000 livres to make peace with de Thoissy. He persuaded Juan de Lascaris-Castellar, Grand Master of the Knights of Malta to pay 120,000 livres for St Kitts, St Croix, St Bartholomew and St Martin. Although de Poincy was immediately appointed governor, Fra de Montmagny was soon appointed in his place. However, when Montmagny arrived, like with de Thoissy, de poincy refused to step aside, and Montmagny was obliged to settle down of the north of St kitts where he lived until his death in 1657.
In 1653 the French king further entrenched the authority of the Knights of Malta on the four islands, retaining sovereignty over the islands with 1,000 crowns to be apid on the accession of each new french King.
De Poincy died at the age of 77 on 11 April 1660. he was a Bailiff Grand cross of the Knights of Malta and Chef d'escadre of the French Brittany Fleet. he was buried in Basseterre, probably in the grounds of what is now St George's church. He is credited with turning Bassterre into a successful Caribbean trading port. The annual Saint Kitts carnival troupe, Les Actors, are people descended from a troupe of Acrobats from Africa whom he had imported to Saint Kitts as his slaves and personal performers for parties at La Fontaine.