He was expelled from the Liberal Party by Sir Wilfrid Laurier in 1907 and he resigned from his seat the following year to run for Ligue nationaliste canadienne in the 1908 Quebec election.

In 1910, he was instrumental in achieving passage of what came to be known as the "Loi Lavergne", the first language legislation in Quebec, which required the use of French alongside English in tickets, documents, bills and contracts issued by transportation and public utility companies.

Lavergne opposed the use of conscription during the First World War, was an ardent defender of French language rights outside of Quebec and lobbied for more French-Canadian participation in the federal civil service.

Though never firmly proven, there is a substantial amount of conjecture that Armand Lavergne was the illegitimate son of Prime Minister Sir Wilfrid Laurier.

[2][3] Marcello Di Fruscia portrays Lavergne in episode 15 of season 13 "The Trial of Terrence Meyers" (February 10, 2020) of the Canadian television period detective series Murdoch Mysteries.