In 1714, the French Crown once more granted the privilege to run the Mille-Îles Seigneury to one of its subjects whose mission was to colonize the region. Thus, Jean Petit and Charles-Gaspard Piot de Langloiserie, the husbands of Marie-Charlotte-Élisabeth and Marie Thérèse—the two daughters of Sidrac Dugué—became the new overseers of the Seigneury. Charles-Gaspard Piot de Langloiserie died in 1715, and Marie-Thérèse de Langloiserie, his widow, was left with a great number of young children and the responsibility of supervising Sainte-Thérèse Island as well as the Mille-Îles Seigneury.

Marie-Thérèse de Langloiserie lived in Montréal where she also brought up her children. In 1730, the first colonists, the Charbonneau family and their companions, were lead by her to the Mille-Îles Seigneury. These were the beginnings of the colonization of the Seigneury and its growth in population. Marie-Thérèse de Langloiserie died in 1744.

One of her daughters, Suzanne Piot de Langloiserie (1700-1769), married Jean-Baptiste Céloron De Blainville and took over from her mother in the task of colonizing the Mille-Îles Seigneury. Between 1745 and 1755, Suzanne De Blainville was personally taking care of all the land grants. Later, in 1755, she decided to get the help of Joseph Filion of Terrebonne who administered the lands together with her until her death in 1769. After Suzanne De Blainville's death, large tracts of the Seigneury's land were cultivated, extending from the Grande-Côte road and the Dogs River to the Cachée River, including the north shore, the south shore, and the Bas-de-Sainte-Thérèse.

Suzanne and Jean-Baptiste Céloron De Blainville had one son, Louis, and three daughters, Marie-Anne Thérèse, Marie-Hypolite and Louise-Suzanne. Marie-Anne Thérèse (1731-1806) married Sir La Marque; and Marie-Hypolite (1735-1810), Sir Hertel.

As of 1769, the year when her mother, Marie-Thérèse —better known as Thérèse De Blainville— was personally in charge of the Mille-Îles Seigneury, most of the legal documents bear Thérèse De Blainville's signature. She died in 1806, leaving behind a well-developed and established Seigneury. Her sister, Marie-Hypolite, only came back to the Seigneury in 1780. She had taken up domicile in the Hertel Manor where she stayed until her death in 1810. The Hertel Manor stood near Philippe Street, at the intersection of Grande-Côte road and the Canadian Pacific railway.

Sir Hertel, the last colonizing lord of the Mille-Îles Seigneury, died in 1817. At that epoch, the Mille-Îles Seigneury was called the De Blainville Seigneury. The part of the De Blainville Seigneury which later was to became Rosemère was transferred to William Clauss, the attorney general of Upper Canada. After William Clauss died in 1826, three attorneys, John Oldham, John Hettrick and David Morris (1823-1909) took turns administrating the Seigneury.

In 1837, the Rebellion lead by the French Patriots against the British broke out. The Patriots of Saint-Eustache set Porteous Bridge (predecessor of Marius Dufresne Bridge at Bélair Island) on fire with the intent to slow down General Colborne and his troops. An English garrison had even stationing itself in Overing House—also called Twin Chimneys—located on Grande-Côte Road.

In 1861, David Morris, the attorney of the Clauss family, bought the family's succession rights and became Sir Morris for the locals.