 He was elected to the legislative assembly along with Henri Bourassa and re-elected in the 1912 Quebec election; Lavergne served as the party's only member from 1912 until leaving office in the 1916 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.In 1904, he had married Georgette, the daughter of Philippe-Honoré Roy, a member of the Quebec assembly.