CDG to Gare de Lyon to Montpellier

Where is your Norse Atlantic flight coming from?

My understanding is that the RER station is right there at Terminal 3

"right there" may be a bit of an exaggeration, as it still looks like about a 300 yd walk to get there.

The RER B with a change to the D will get you to Gare de Lyon in 50 minutes, so it seems if you can make it to the RER station by 6 pm, you should be OK.

I'll be flying in from LAX so I have to go through passport control. That's why I mentioned being at the front of the plane, so I'll be one of the 1st ones out (somebody said that would make a difference, but I really don't see how, as so many planes will be arriving).

Well, I just realized that I have (as do most of us!) a biometric passport. Evidently that speeds things up.

Yes, passport control can take anywhere from 10 minutes to an hour. Being in the front of the plane puts you ahead of the other people on you plane but you are correct that people from other flights might be in front of you. If you are flying in business class, there might be a dedicated line for business class passengers for passport control. That is typically a game changer for getting out of passport control quickly but passport control once took me over two hours flying into Paris from Ireland and I was in business class. There was some sort of disturbance and they shut down the line I personally think that you are cutting it very close but you will make probably it if your flight is on time, passport control takes less than an hour, and there are no significant train delays. The trip by RER is about 50 minutes not including time needed to purchase your ticket. The usual advise is to allow 4 hours for the transfer that you are making but you do not have checked luggage which helps. Good luck, but I suggest being ready to take a later train.

I have a biometric passport, so I would assume that would save time. I also have purchased my RER ticket on my phone so that's taken care of.

I say go for it but be prepared with a back up plan like a later train. I have a biometric US passport and it has never gotten me through passport control at CDG any faster but I am never really rushing through the airport.

By the way have either of you taken the RER B to the RER A to Gare de Lyon?

Do you have to go upstairs to change trains, or is the platform right there?

The track for the train to Gare de Lyon is across the platform from the southbound RER B. That's why the change to the A from the B is easier than the change to the D. You'd have to go upstairs if you were catching the RER towards Etoile. Also look at the RATP site or CityMapper if you have not already to see which cars of the train they suggest will be best for your trip.

That’s cutting it awfully close, considering it takes about 50 minutes to get from CDG to Gare de Lyon. I would use seat61.com to get familiar with the station. It is big, with multiple levels. As you arrive on the RER, watch the RATP app to see if it tells you which hall & platform you need to connect to. Keep a close eye on your bag and be pickpocket proof.

The post from JHK is referring to changing from the "B" to the "A" at Le Halles station. That is indeed cross-platform.

This person is way incorrect: The train from Paris Gare de Lyon to Montpellier usually takes 5 hours 11 minutes on average

It's 3.5 hours or less. I take it quite often.

Now to the OP question: dicey but doable. If it doesn't work, just open your phone and stay in a hotel near the Gare de Lyon. There are about 50. There are trains the next morning, the first at 6:42. I wouldn't take the night train, my idea of misery. I took plenty in my youth before the invention of the TGV.