Wellllllllll,

www.ratp.info/informer/anglais/index.php# claims that it COULD be done in only 53 minutes from CDG platform to Monparnasse platform. This assumes that you can get your checked bags (HAH!!), transit to the CDG RER station, purchase your tickets and be on the correct platform by 9am. There are often many bozos in line trying to buy tickets (in English) to who-knows-where (Antarctica, perhaps). Phewwwwwww.

Using www.ratp.info/informer/anglais/index.php# I input the departure point as Aeroport Charles de Gaulle 1 - RER and the destination as Gare Montparnasse - SNCF

Judging by the scheduled arrival time I assume that the OP is arriving on UA's IAD-CDG flight that arrives at CDG Terminal 1. There is a lengthy walk from the plane to the immigrations counter and often a wait. Then there is often a long wait for checked bags. Then there is a project finding the correct elevator to take you to the lower level where the shuttle bus will take you to the RER station. Then there is the ticket buying circus. It is all remotely possible if the pax sit up front in C or F and walk/run, and the bags arrive quickly. But this is not very likely or easy.

Note that this plan requires the initial transit to the RER (described above) and a transfer from RER to Metro. The Metro ride should be very short.

To the OP: Do you speak some French? Have you ridden the RER or Paris Metro before?