He was born Pierre-Alphonse-Séverin in Saint-Rémi, Canada East, the son of Léon Lachapelle and Rébecca Lanctôt, and was educated at the Petit Séminaire de Montréal.
He contributed to the monthly L’Union médicale du Canada, also serving on its editorial board, and wrote a science column for the Revue canadienne; he also was the first editor for the Journal d’hygiène populaire and the magazine La Mère et l’enfant.
He helped establish the Gouttes de Lait program at Montreal with the aim of reducing infant mortality.
Lachapelle was first elected to the House of Commons in an 1892 by-election held after Alphonse Desjardins was named to the Canadian senate.
In 1880, Lachapelle said "There is one particularly unsettling belief that is particularly widespread and deeply rooted in our families because it is shared by medical practitioners, and that is that there is no point in treating children's diseases.