After the fall of Napoleon's empire, de la Croix resumed his studies and was ordained in Ghent by Bishop Dubourg of Louisiana, whom he later followed to the United States along with several other seminarians and some Flemish workmen.
After the arrival of Father Rosati, president of the new seminary, de la Croix went to Florissant, Missouri on 3 December 1818.
With the help of the newly arrived colony of the Society of the Sacred Heart, he worked to prepare the way for Pierre-Jean De Smet and the other Jesuit missionaries, who came to Florissant in 1823.
When Father Van Quickenborne, S.J., arrived with his eight companions, de la Croix had nearly completed a brick church, started a farm, and opened a missionary field for the work of young Jesuits.
Having been appointed to St. Michael's parish in lower Louisiana, Father de la Croix prepared a convent for the Society of the Sacred Heart, in which they opened a boarding school in 1828.