Chagnon was first elected in the riding of Saint-Louis in the 1985 election and was re-elected again in 1989. He was then elected in the new riding of Westmount–Saint-Louis during the 1994 election.
In the 1985 election, Chagnon was elected to represent Saint-Louis. He was subsequently re-elected for a second term in 1989.  He then won the newly created Westmount–Saint-Louis riding in 1994.