Paris, Chartres, and Mont Saint-Michel in a day?

It will be dark out by 4.30 - 5 pm at latest.. consider that when trying to figure out timing.. plus fact in off season the train/bus connection to MSM is infrequent.. so the likelihood of this working is about nil.

Save the better odds of good light for Chartres, rather than MSM. However, Chartres is beautiful even on a rainy overcast days – the only way I've seen it in three visits. The church at MSM is brighter, without a lot of stained glass, so more light gets in.

To answer your transit question, as I remember, bus service to/from MSM is much more frequent from Rennes (at least in shoulder season).

Finally, I hope you're staying overnight at MSM? Definitely stay on the island, and spend some time outside of the walls after dark. It makes for stunning views and photographs

Not possible to do in one day. It takes too long to get between each of these places (except for between Paris and Chartres, that is).

Rome2Rio lists Paris to Mont St Michel at nearly 5 hours so I don't see how this would in any way be practical, especially in January.

I don't want to squash any dreams, but you might want to break this one into 2 days. I can't imagine you seeing these sites in the limited amount of daylight that is available in January (keep in mind that France is pretty far north, so winter days are quite short).
Both worthy sites, but each deserves at least a day.

Thanks for all your advice. I will split them into 2 separate days.

I discovered after posting my question that the train from Paris to Rennes (the fast train) does not stop in Chartres. To do so would mean taking a train that stops at every station between Chartres and Rennes.

I agree with the others - with the limited daylight in January (and it is substantial), you won't be able to do both in one day when you factor in that you are staying in Paris.

Staying overnight on MSM is a unique experience, so that would be my recommendation, then head to Chartres the following morning.

Lisa