Departure from Paris Montparnasse station:
TGV to Rennes (2h00) then TER connection from Rennes to Pontorson station, where a shuttle bus awaits you. Possibility of taking the coach from Rennes station directly to Mont-Saint-Michel (1h20). TGV to Dol de Bretagne (2h40) then TER connection to Pontorson station (15 min)
From Saint-Lazare station in Paris:
TGV from Paris Saint-Lazare to Caen (2h08) then shuttle from Caen to Pontorson.