From Paris Montparnasse station to the Gare Du Nord by Metro

A service of underground commuter trains, known as RER trains, can speed travellers from the Gare De Lyon to the Gare Du Nord in less than 10 minutes, but there is no such service between the Gare Montparnasse and the Gare Nu Nord on the other side of the city centre.

So if you don’t want to take a taxi, the taking the Metro between these two stations is the quickest public transport option.
It looks straightforward, as Metro Line 4 provides a direct link between Montparnasse and Gare Du Nord (direction Porte de Clignancort), but it is not step free, so isn't suitable for those with restricted mobility, or if you want to avoid having to carry luggage.

Taking the Metro is also at least 5 x cheaper than taking a taxi and almost certainly faster too, despite it being a 14 stop journey; so you will be on the train for around 20 - 25 minutes.
Though if time is your side bus line/route 91 is now the core inter-station bus service in central Paris.
It heads to the Gare Du Nord from this stop accessible via Hall 2; which exits on to the Place de Cinq Martyrs du Lycée Buffon
And from *this stop

If you take the metro, line 4 is also the 2nd busiest Metro line 4 in Paris; so try to time it so that you’ll be using outside of rush hours.
The typical total transfer time, from stepping off a TGV at Gare Montparnasse to being at the Gare Du Nord will be around 35 - 40 minutes.

However, if you are connecting to the Eurostar and have Standard or Standard Premiere tickets, you will need to allow a minimum of another 30 mins to pass through check-in etc.
So when booking connections into Eurostar trains avoid any journey with less than 1hr 20 mins between the scheduled arrival at Montparnasse and the departure time from Gare Du Nord

Step One: Walk ahead to the front of the train and exit on to the main concourse (the only exit from the platform/voies).

Many TGV trains that arrive at Montparnasse can be 20 coaches long, so if your reserved seat is towards the rear of such a train, it will take more than 5 mins before you step on to the main concourse at the station.

Step Two: When you have reached the concourse (where the main departure board will be hanging from the roof), keep going ahead of you until you are in the Hall 1 area of the station.
You will see two banks of escalators and stairs, both of which connect this Hall 1 to the entrance to the Metro station, which is located two levels below.

Though a tad annoyingly both escalators can be switched so that they don’t allow people to descend.
When that is the case and you have luggage it can be worth seeking out the somewhat hidden lifts/ascenseurs.

Step Three: The Metro station is located at the foot of the atrium which houses these stairs and escalators.
When you have reached the foot of this atrium, there are a few steps UP to the Metro entrance.

Step Four: Buy a [t+ ticket]https://www.ratp.fr/en/titres-et-tarifs) at the counter or machine: the machines have excellent English translation, but look out for the easy to miss 'confirm your purchase' button.
Though at busy times and/or when the staffed ticket office is closed queues can build up for the ticket machines; it can take more than 10 mins to buy a ticket.
Or you can buy a single journey t+ ticket on your phone.

(Your end-2-end train journey ticket will not include the Metro).

Step Five: The Line 4 trains depart from the part of Montparnasse – Bienvenüe metro station that is located furthest away from the entrance to the Metro.
The longest passage way between the entrance to the Metro and Line 4 houses a travellator.

At the end of the travellator look out for the Line 4 signs to trains heading to Porte de Clignancort, don’t follow the signs for trains heading to Marie de Montrouge.

You will need to go down a set of stairs to access the platform/voie which the trains heading to Porte de Clignancort will depart from; there is no escalator or lift access.

Step Six: On arrival at Gare DU Nord you will have to use a short flight of stairs, located towards the front of the train, to access the exit from the Metro.

Step Seven: Having exited the Metro station there are two routes available into the station, heading to the left or going straight ahead.
If you head left down the passage way, after a minute on the right you will see stairs and an escalator, which can travelling up or down, this exit from the passage way leads up to the concourse which most of the long-distance trains use.

From this exit, over to the right will be the voies (platforms/tracks) which the Thalys trains depart from, so if you follow the signs from the metro exit, which point to the Thalys trains, you'll be using this access to the concourse.

Most of the TGV trains to other destinations in France also leave from this part of Gare Du Nord, as do the Eurostar trains, but the access to Eurostar departures is in a different part of the station, which is above this link between the Metro and the concourse, but there is alternative, more obvious route available.

Connecting to Eurostar departures

In this view taken from above, the exit from the Metro station is in the far left corner, to the right of the blue poster - the escalator in the middle of the picture is the main link up to the main departure concourse.
You will exit into an atrium, pictured below, from where you will see the main concourse at the Gare Du Nord above you.

Take the escalators or lifts up to the main concourse; so if you follow this route and everything is in working order, you will have step-free access from the metro exit to the station concourse which is where the mainline trains depart from.

Walk ahead of you to access the part of the concourse adjacent to where the TGVs and Thalys trains depart from, you'll see them ahead over to the right.
Because the trains are over to the right, it's easy to assume that you'll need to head towards them, but you will have to go up another level to access Eurostar departures; the ‘Hall du Londres’ and the escalators and lifts (ascenseur) leading up to it are over to the left as you walk towards the trains.
They are located on the opposite side of the concourse to voies/platforms/tracks 14 - 15; look out for the Union Jack on the rear wall of the station concourse.

However, if you are connecting to the Eurostar and have Standard or Standard Premiere tickets, Eurostar is currently recommending tghat you should be at the entrance to the Hall Du Londres 90-120 mins before departure; and that Business Premier ticket holders should be there 45-60mins prior to departure.
The departure gates will close 45 mins prior to departure for Standard Class And Standard Premier ticket holders; and 15 mins prior to departure for Business Premier ticket holders.