The urban agglomeration of Longueuil was created on January 1, 2006, as a result of the de-amalgamation process brought upon by the Charest government. It encompasses all the boroughs that were merged into the previous city of Longueuil and still retains the same area as that mega-city.
The urban agglomeration of Longueuil is coextensive with the territory equivalent to a regional county municipality (TE) and census division (CD) of Longueuil, whose geographical code is 58.
The 2000–2006 municipal reorganization in Quebec resulted in large-scale amalgamation of smaller municipalities in Quebec into larger cities. It was undertaken by one administration, but was modified and partially undone by its successor.
The first phase involved many amalgamations from late 2000 until 2003, undertaken by the Parti Québécois government of Québec, headed by Premier Lucien Bouchard and his successor Bernard Landry. The most significant amalgamations, involving the largest cities in Quebec, mostly occurred on January 1, 2002. Some of the mergers were unpopular, and this became an issue in the April 14, 2003 Quebec election, in which the victorious Quebec Liberal Party led by Jean Charest campaigned on a promise to allow residents the right to choose to de-merge and reconstitute their former municipalities.