L'Allier was appointed to the Cabinet in 1970 and served as Minister of Communications until 1975 and as Minister of Cultural Affairs from 1975 until 1976.
L'Allier ran as the Rassemblement populaire candidate for Mayor of Quebec City in 1989. He won against Progrès civique de Québec candidate Jean-François Bertrand and was sworn in as the 38th Mayor of the city. He was re-elected in 1993 and 1997.
His accomplishments include:
The revitalization of the Saint-Roch neighborhood;
The erection of a monument that commemorates the 30th anniversary of French PresidentCharles de Gaulle's official visit to Quebec City in 1967;[3]
The merger of the Quebec City government with twelve other surrounding municipalities, as a part of the municipal reorganization of 2001–02.
L'Allier co-founded the Renouveau municipal de Québec and was re-elected as mayor in 2001 against Action civique de Québec candidate and former anti-merger crusader Andrée Boucher. On July 13, 2004, he announced that he would not run for re-election in November 2005 and retired from politics.