At the time of the Second World War, Rosemère was already a flourishing community. Rosemère was home to a Red Cross branch office, and a citizens association was founded with the aim of obtaining better services from the Town of Sainte-Thérèse. Rosemère did not yet exist as an independent town, as it was still under the jurisdiction of the municipality of Sainte-Thérèse.

Services such as snow removal and swamp evaporation were not up to par. To remedy this situation, the new Rosemère Citizens Association asked for the right to create their own parish. Carolyn Owens, who later became a municipal council member, had already at that time been an active member of the Association.

On January 1, 1947, the residents of Rosemère won their case and the Parish of Rosemère separated from the Parish of Sainte-Thérèse (founded in 1845). Rosemère's first City Council comprised six councillors—Alphonse Couture, Hector Labelle, Hubert Maisonneuve, Ernest Gilmour, André Robitaille and Jack MacDonald—and was headed by the mayor, Raymond Perrault.

The same year, a fire completely destroyed one of the homes in Rosemère, leaving the homeowners in utter destitution. Berthe-Yvonne Clément, called Beppy, an active participant in community life and welfare, collected money and clothing for the stricken family. Her humanitarian gesture was welcomed by all, and, as a result, a welfare service was called into existence.

Hector Labelle was another resident actively involved in various community projects. A profoundly religious man, it is thanks to his efforts that a priest from Sainte-Thérèse would come to celebrate mass for the Catholic community during the summer months. The Sainte-Françoise-Cabrini Parish is established September 4, 1947.

Over the years, a number of other groups and associations emerged, such as the Boy Scouts, the Brownies, l'Atelier de théâtre, the Horticultural Society, and many more.

Rosemère continued to grow and was soon expanding beyond the boundaries of a simple parish. In the Late 50’s, Mayor H.J. Hemens wanted to approach the provincial government to apply for municipal status, without having to forfeit government parish grants. A council meeting was held, which was also attended by a very large number of residents. The council obtained authorization and could go ahead with applying for municipal status. The parliamentary assembly granted the mayor his request and, on February 6, 1958, Rosemère was officially given the status of a town. At that time, the population had reached the 5,500 mark.