I was looking for something else on the Man in Seat 61's website and saw this, thought I would post for reference --
Mont St Michel has no train station, the nearest station is Pontorson-Mont St Michel which is about 5 miles away.
Option 1, cheapest & easiest, but slower, total journey from Paris around 4h30: Take a regional train from Paris Montparnasse to Pontorson-Mont St Michel then an integrated connecting bus to Mont St Michel. You can book from Paris to Mont St Michel as one transaction at www.thetrainline.com or www.sncf-connect.com, you'll get a through ticket covering both the train and the connecting bus. There aren't many departures per day, typically one morning service out and one late afternoon service back, allowing day or weekend trips from Paris. If these don't suit you, buy a ticket to/from Pontorson-Mont St Michel station and use a taxi between the station and Mont St Michel.
Tip: When selecting your seating options on www.thetrainline.com, don't panic when it says 'no places available' against the bus (or in French 'car'). That just means there are no allocated seats, you can sit where you like. It does not mean the bus is full! And yes, it's a bus between Pontorson and Mont St Michel, not a train, whatever it looks like in the data.
Option 2, faster, total journey from Paris around 3h: Take a high-speed TGV from Paris Montparnasse to Rennes in 1h30 then a connecting Keolis bus from Rennes to Mont St Michel in around 1h10. From April to September there are 3 buses a day between Rennes & Mont St Michel in each direction, see keolis-armor.com. Book a suitable train to/from Rennes at www.thetrainline.com or www.sncf-connect.com. Buy the bus ticket at Rennes bus station, see keolis-armor.com for prices & more info.
Option 3, between May & September there's also a daily bus from St Malo to Mont St Michel, journey time 1h20, see keolis-armor.com for dates, times, prices & more info. St Malo is linked by ferry to Portsmouth with www.brittany-ferries.co.uk and with Paris by train.