Mont St Michel - what to do with luggage?

I suggest you contact the Auberge Saint-Pierre for advice. We did not stay on the island for our visit, but we did see bags being handled at the shuttle stop (before new causway was finished) for arriving hotel guests.

If you find no better solution...I know it's a risk, but honestly it is a massive parking lot filled with tourists; surrounded by meadows and sheep. Probably every car in the lot has some luggage in the trunk. Obvious a risk, but if you don't find a nanny bag location or the hotel has no suggestions, I'd just make sure everything you don't need is locked and hidden from sight in the trunk (before you arrive to the parking lot); have your valuables and the stuff you are taking overnight in the backseat to just grab and go; no opening the trunk, and go enjoy your stay.

Here's a forum conversation from last year...
https://community.ricksteves.com/travel-forum/france/luggage-storage-options-mont-st-michel-nannybag

Thanks for your prompt replies. I did contact the hotel and they said there were no luggage options on the island and when I asked if they considered the parking lot secure, they said "At this time we didn't had any problem with the parking", I assume meaning they've had no complaints. Your replies and the additional post from last year (thanks!) help me alleviate concerns. We will do as suggested and hide the luggage as best we can (a little harder to do with SUVs then sedans with trunks).

I am NOT endorsing leaving visible luggage in a car, but did you inquire as to whether the on-island hotel has its own parking, separate from the giant array of hundreds of spaces? I believe that many of the "off-island" chain hotels along the MSM entry-"strip" do have separate parking, with a gate that presumably "guests" can operate. Just an idea to consider.

That is true, there is a coded gate where we are instructed to park. Having not seen the parking area, I assumed this could just simply be a mechanic "arm" that is raised/lowered with the code--not necessarily a separate "secure" parking area. Normally I would never leave valuables in our rental car but I see no other feasible choice.

I don't know how your specific hotel would work. However, we stayed last month at Hotel Mercure, one of those close to the bridge. The code the hotel gives you allows you entrance to a separate area close to the bridge. We then drove to our hotel and parked right next to the hotel. The parking was not part of the huge parking lots which are off to the east of the coded gate.

IMHO, We did not feel that we were missing out on something special by not staying on MSM this time, since we had done that on a previous trip. We actually enjoyed the river and dam area directly behind our hotel much more than walking past the tacky shops on MSM. MSM is lovely to look at and walk around below the walls, but extremely touristy once you enter.

What KD said. And do stay on the island. No one who has actually stayed there would say it isn't worth it. Only those who have never stayed on island.

Very much appreciated, I have more confidence and peace with this decision. Thanks!