He ran for election as Mayor of Trois-Rives, then known as Boucher, in 1978, but lost against Elphège Desrosiers. Mongrain ran again in 1981 and defeated the
incumbent. He was re-elected in 1985 and won every subsequent election, running unopposed in 1989, 1993, 1997, 2001, 2005, 2009 and 2013. In 2017, he won re-election with 247 votes (62.22%) against Joël Francoeur (77 votes) and Claude Doucet (73 votes). In 2021, he retired after forty years as mayor. During Mongrain's tenure (in 2007), a fire department that services Trois-Rives, as well as
Grandes-Piles and
Saint-Roch-de-Mékinac was established. Lucien Mongrain ran a debt-free administration that oversaw an increase in property value from $3.6 million in 1978 to $119 million in 2012 and in budget from $57,236 to $1,487,971, as well as infrastructure investments. ==Footnotes==