Paris to D-Day Beaches to Mont St. Michel in one day in June?

It is doable. But kind of forcing it. You are driving to Normandy from Paris. The visit to DDay area and the American Cemetery is mostly walking and standing. And then driving to Mt St Michel the same day.

I would stay in Bayeux for a couple after the DDay tour then go to Mont St Michel for that one night

I'd say really forcing it - I'd rent a car in Paris and plan to leave very early in the morning to see the D-Day beaches. You may get sucked in and stay longer than you had planned! I'd plan to arrive at Mont St Michel around sunset as the view from further away is awesome. After dropping off the car, so you plan to take the train and leave from CDG? We previously rented a car from CDG and it was easy to jump on the freeway towards Normandy as well as return the car to CDG 2 weeks later.

In my humble opinion, these two things (MSM and D-day sights) are not doable in one day or even two. It is nice to read guide books and recommendations and imagine all the places you'd like to see. It is quite another to look at the geography and try to fit as many places into limited days as possible. MSM (one small location far from Paris) is a very long stretch to do in one day from Paris. The D-day beaches (many locations spread out over an hour plus driving alone between them) are not doable in one day in many peoples' opinions on this forum. Combining both in one day is simply a very difficult exercise to get nothing more than maybe "photo ops" in each place without experiencing them at all. And even at that, you would do nothing but spend a very long day just struggling with transportation to get from place to place. Allocate more time, and/or pick your priorities. Or just plan on seeing "everything" over multiple trips.

I would train to Caen in the afternoon, rent a car and see the Bayeux tapestry, and have a mellow evening night in Bayeux, then do D-Day sites and drive to Mont St. Michel, arriving late, and spend the night there, then return car and depart the next morning. Or, I arrived at MSM at 5p, stayed till 7 when it closed, ate dinner there, and drove back to Bayeux by 9:30p, and departed Caen the next morning early...
You could certainly do all this in one day, but I would still take a train out of Paris, an early one ...

Is it physically possible? Yes.

Would I recommend this to anyone I actually liked? Absolutely not.

Taking the early train to Caen or Bayeux and renting, or renting in Paris and then driving - either way, you won't get there till nearly noon. Then you'll be battling traffic in unfamiliar areas on unfamiliar roads and vying for parking spaces to only spend a few fleeting minutes at a site before moving on and repeating the process. That is NO way to see these sites. If you really want to spend only half a day touring the D DAY sites, sign up for one of the tours, like the one by Overlord. They'll do a much better job than you can do yourself in that time. Personally I think the sites need much more time than that to fully understand and appreciate them.

If you drive to MSM that evening, get a hotel near, but off, the island, and visit it early the next morning. Then head off to the Loire. It will take at least 3.5 hours to get as far as Amboise so you won't get there till later in the afternoon ( assuming stops for gas, lunch.) How many days are you planning on spending in the Loire? Assuming you rent from a rental agency with an office there, you can drop the car in Tours at the St Pierre des Corps station and take the train back to Paris.

Definitely agree, don't try to do it all in one day. But do stay on Mont St Michael its self, its an amazing experience, and you get to see the island once all the day trippers have gone. Would never go during the day, far too busy. Not nice.

Not doable at all. You can't even see all five D-Day beaches in one day.

Our day trip went like this. We left Gare St Lazare very early in the morning and arrived in Caen around 8:00 am. We rented a car and drove to Pegasus Bridge, Ouistreham/Sword Beach, St-Aubin/Juno Beach, Juno Beach Centre at Courseulles-sur-Mer, Canadian Cemetery at Beny-sur-Mer, and Arromanches/Gold Beach. We had been intending on continuing to Omaha Beach, but by then it was 5:30 and we had to drive back to Caen to catch our train back to Paris.

Even if you wanted to focus just on the American beaches, Utah and Omaha are not adjacent and you would be looking at most of a day to see the two American beaches, just as we spent a day seeing the three British/Canadian beaches.

Mont-St-Michel is yet another 2 hour drive from either US beach.

I agree with CJean. If you drive, park, see a beach and move on, you are absolutely cheating yourself. It just looks like a beach. You might as well look at photos. On the other hand, with a good tour (even half day) you will hear the strategies, where the Germans were, where the tide was, what some individual soldiers did where, what went right, what went wrong.

If you merely want a "check-off list" to say "we did that" ... then drive by. If you want a deeply moving experience, take a tour. Worth it. TripAdvisor has plenty of reviews of good tour guides and so does Rick Steves.

You wouldn't be doing one of the full day or half day tours. Their times wouldn't fit your schedule. If you left Paris early, caught a train to Caen and rented a car there, I think you could spend four or more hours - self guided - at D-day beaches before heading south to MSM. It's about 1 1/2 hours driving time from Bayeux to MSM. The time in the area depends on whether you want to arrive in time to see the Abbey before closing or plan to spend the night and see sights in the morning?