Obviously Mechs can work on planets without atmosphere, but does that require retooling at all, or can you just bung some oxygen containers in one and drop it off? I’m writing a fanfic and I’d rather not have the main character explode. 
Definitely. I seem to recall mentions in the game literature about battles fought on moons, etc…
I think the battle where Phelan Kell gets captured was on an airless world, if memory serves… (don’t remember which novel though).
They have printed rules in the past about mechs fighting in total vacuum, moons, etc. and if I recall correctly, and it’s been many years, once you score an internal hit that whole area shuts down. So if you’re on a moon and someone scores a single internal hit on your left arm then the left arm is useless for the rest of the game.
Marc
Christ, they should be, otherwise I’m heading to Costco to return mine.
I have no idea how canonical this is, but there was a series of missions in Mechwarrior 2: Mercenaries in which you were fighting on a gigantic comet that was being towed.
The idea was that the comet was full of ice that could be used to supply water to a desert planet. Your opposition was hired by the water importers who faced bankruptcy.
Hmm, I think I should have made my question a little clearer. I know Mechs can be used outside of atmosphere, but would maintenance be needed to make them capable? Could any old Mech just be dropped onto a moon, or would extra work need to be done on them in order to allow them to run at all?
I’m pretty sure the cockpit is sealed, since on a low-grav world ejecting might achieve escape velocity, but there might be modifications needed to make sure the Mech can stand up to the air pressure, and so on.