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.

In 2012, Longueuil mayor Caroline St-Hilaire proposed that the Urban agglomeration of Longueuil leave the Montérégie and become its own administrative region.

Longueuil merged on January 1, 2002 with the communities of Boucherville, Brossard, Greenfield Park, LeMoyne, Saint-Bruno-de-Montarville, Saint-Hubert, and Saint-Lambert. These cities became boroughs of the Longueuil megacity. Saint-Lambert and LeMoyne combined to become one borough called Saint-Lambert/LeMoyne. The former city of Longueuil was renamed Le Vieux-Longueuil borough.

The former city hall of Brossard, became the city hall for the new city of Longueuil.

On June 20, 2004, the former boroughs of Boucherville, Brossard, Saint-Bruno-de-Montarville and Saint-Lambert voted to demerge from Longueuil to reconstitute themselves as municipalities on January 1, 2006. The rest of the city stayed intact.

The departure of Saint-Lambert from the city of Longueuil resulted in the immediate disbanding of the Saint-Lambert/LeMoyne borough. LeMoyne's small population and territory did not allow it to become a borough of its own. In 2005, the population of LeMoyne was given the choice to pick a new borough between Le Vieux-Longueuil, Saint-Hubert and Greenfield Park. Le Vieux-Longueuil ended up being the winner and amalgamated LeMoyne into its borough on January 1, 2006.

Following the demergers, Longueuil relocated its city hall from Brossard to Saint-Hubert, where it is still located.

According to the Act respecting the exercise of certain municipal powers in certain urban agglomerations, the cities and boroughs of the urban agglomeration of Longueuil are structured as follows:

• Ville de Longueuil
  • Borough of Le Vieux-Longueuil
  • Borough of Greenfield Park
  • Borough of Saint-Hubert

• Ville de Boucherville
• Ville de Brossard
• Ville de Saint-Bruno-de-Montarville
• Ville de Saint-Lambert

Cities and Boroughs of Longueuil Agglomeration (2016 census)

	City/Borough	Population	Pct (%)	# of Representatives	# of council votes
	Boucherville	41,671	10.0%	1	1.76
	Brossard	85,721	20.7%	1	3.21
	Longueuil	239,700	57.7%	6	10.63
	Vieux-Longueuil	138,550	33.3%
	Saint-Hubert	84,420	20.2%
	Greenfield Park	16,735	4.0%
	Saint-Bruno-de-Montarville	26,394	6.3%	1	1.13
	Saint-Lambert	21,861	5.3%	1	1
	Total	415,347	100.0%	10	17.73

Under this new system of municipal organization, the agglomeration city and the reconstituted cities (in this case, Boucherville, Brossard, Saint-Bruno-de-Montarville and Saint-Lambert) share powers and responsibilities. The urban agglomeration is headed by an agglomeration council which exercises these agglomeration powers.

Historical Census Data - Urban agglomeration of Longueuil, Quebec

Year	Pop.	±%
2001	371,934	—
2006	385,533	+3.7%
2011	399,097	+3.5%
2016	415,347	+4.1%

Language
Mother tongue from 2016 Canadian Census

Language	Population	Pct (%)
French only	295,710	71.96%
English only	29,350	7.14%
Both English and French	4,695	1.14%
Other languages	73,520	17.89%

Highways and numbered routes that run through the municipality, including external routes that start or finish at the county border:

• Longueuil City Council
• Municipal reorganization in Quebec
• Urban agglomerations of Quebec
• Longueuil
• List of mayors of Longueuil
• Boroughs of Longueuil
• List of regional county municipalities and equivalent territories in Quebec

• Bill 6 (An Act to amend various legislative provisions respecting municipal affairs) as applied to the Ville de Longueuil
• Reconstituted municipalities (in French)
• CRÉ (in French)

45°32′N 73°31′W / 45.533°N 73.517°W / 45.533; -73.517