This is probably pretty straightforward, but my wife and I (and we re not young!) need to get from Gare St Lazare to Gare Montpasse 1 and 2 mid-morning on a Saturday in a couple of weeks. Time does not seem a problem - we have an hour and a half and we would prefer not to have to use a taxi. Any advice would be appreciated.
http://www.ratp.fr/itineraires/en/ratp/recherche-avancee
Take the M13 direction Châtillon Montrouge 8 stops (11 minutes). Including getting off a train, walking to the metro, buying a ticket, taking the metro, walking to another train, allow at least 30 minutes in total. A 1 way ticket (known as a T+ Ticket) is €1.80 each, the ticket machines have an english language option. A general how to guide is available here: http://www.ratp.fr/en/upload/docs/application/pdf/2014-10/mode_emploi_t.pdf

Hi,
You can take subway : line 13 or line 12, it's direct. I would recommend line 13, it's faster than line 12.
But, if you have time, the best is to take the bus, line 94 : http://www.ratp.fr/informer/pdf/orienter/f_plan.php?nompdf=94&loc=bus_paris/
With the bus, the ride is wonderful, you go through the Madeleine, Concorde (with the obelisk and Champs Elysées), then the National Assembly and the St. Germain district and finally the Gare Montparnasse !!
What a program for a simple bus ride.
Have a nice trip !
That could work too as it's 22 minutes by bus. OP - if you do this make sure to validate your ticket before you get on the bus. The bus terminates at Montparnasse, so it's hard to get lost or confused.

That's great - thanks so much for your help. One more question - we will each have a reasonable-sized suitcase. Would that be a problem on the bus? Thanks again.

Transferring from the Metro to the train station involves a fair trek. The longstanding blog Paris By Train explains the route, with photos http://parisbytrain.com/gare-montparnasse-photo-tour/

It's only about 20 minutes by bus (The bus terminates at Montparnasse). I always try and uses buses in cities, you see so much more.....
I would take metro line 14 from Saint Lazare to Châtelet and change to metro line 4 there to get to Montparnasse. The advantage of metro line 14 is that it starts at Saint Lazare so you will always have a seat.

Not young either...
With bags...
Not knowing the bus stops..
And Montparnasse is large....
Taxi on Saturday should be reasonable....15-20 euros...
Driver will load your bags...take you to right entrance...
Promptly and with
Much less stress..
Metro is not friendly with luggage....bus has better view but bags a haste to get on and off...
The validation mention means
Swipe your
Train tickets at the small yellow machine on
Your train track
Before you board at Montparnasse
Happy trails