L'Assomption was the result of the merger of the parish municipality of Saint-Pierre-du-Portage-de-l'Assomption (incorporated 1855) with the town of L'Assomption (incorporated 1888) in 1992. L'Assomption is located about 50 km northeast of Montréal, in a loop of the meandering L'Assomption River, just below the confluence with the Achigan River.

L'Assomption takes its name from the Seigneurie de l’Assomption granted to Pierre Legardeur de Repentigny in 1647 and from the river itself, which has carried that name since the early 17th century. Originally, the site of L'Assomption was known as "Le Portage" because it straddled a portage path used as a shortcut across the quasi-peninsula made by the winding course of the river.

Before Joliette came into its own in 1863, L'Assomption was the main town on the road linking Trois-Rivières to Montréal. Its fame was closely linked to Collège de l'Assomption, founded in 1832. The driving force behind this educational project was Jean-Baptiste Meilleur, the first superintendent of education for Lower Canada (1842–55). Canada's seventh prime minister, Sir Wilfrid Laurier, was one of the college's most famous students.

Oasis du Vieux Palais de Justice, the oldest circuit courthouse in the province of Québec, is its foremost historic building.