Traveling late night by car from Mt. Saint Michel to Paris

How late at night do you mean? And why do you need to do it late at night? There is a bus/train combo that you can use to transit from there to Paris so that you don't end up in Paris with a car, which would not be fun.

No less safe than anywhere else you might be driving...do you drive at night where you live? You should be fine if you have a good GPS.
The one caveat I would give is that driving in central Paris is not for the faint of heart. I have done it quite a bit, at all times of the day and night, and it's is extremely challenging even if you know your way around. There are lots of one way streets, choking traffic even late into the night, very tough parking, and streets that go every which way.
I would suggest driving to a point outside of the city, spending the night, returning your rental car the next morning, and then taking public transportation into Paris.
If you must drive into the city at night make sure to study the map in detail so you know where you're going and know where you are going to park ahead of time. Otherwise it could be a very frustrating experience after the long drive when you are tired. Doesn't sound like a good idea to me.

Thank you Valerie

I meant after 10 pm o close to midnight. I would think public transportation would be non existent or scant at that time. I can drive in New York and have driven in Rome during day time so I think I can manage to get to Hotel or Car rental. Thank you for your advise,

Thank you Anita

Yes will have GPS and we may decide to follow your suggestion and park outside.
Thank you for your advise