Rennes

Express buses provide a link between the Gare de Rennes and Mont St Michel.

At Rennes they depart from the bus station which is adjacent to the train station, while at Mont St Michel, they stop at the Tourist Information Office which is located near the end of the causeway, which links Mont St Michel to the mainland.
A shuttle bus runs along the causeway, but allowing a minimum of 25 mins from arriving on the bus from Rennes and being on Mont St Michel is recommended.

The buses don't have a route number and Rennes bus station can be a tad confusing - so to find out which stop the bus will be leaving from, look for Mont St Michel on the electronic departure boards.

The bus journey from Rennes station to Mont St Michel is scheduled to take 1hr 10mins and there are 4 - 5 x buses per day year round in each direction.
So NOT every TGV from Paris has good connections into a bus on to Mont St Michel -  in fact most don't.
More information, including the timetables/schedules from and to Paris is available here.

Note that around 20 - 25 mins is allocated for making the connection at Rennes - and the bus station is only steps away from the railway station.
So plenty of time to buy the tickets or stop and ask for information if you get confused - IF the train from Paris arrives on time.

TGVs from Paris very rarely arrive in Rennes more than 10 mins late, but the advertised connections between the arrival of the train from Paris and the departure of the bus, are NOT guaranteed.
To save time at Rennes, booking combined tickets for the train and bus is recommended and these tickets are sold on Oui.SNCF
Enter ‘Le Mont St Michel’ as the destination – the automated prompt seems to be temperamental.