In Pokemon Let’s Go there are a few reasons that you might want to head back to Mt. Moon once you’ve left for the first time. Unfortunately you cannot go back right away. To get back to Mt. Moon in Pokemon Let’s Go you’re going to need to learn some of the unique abilities in the game.
There are two abilities that can get you back to Mt. Moon. The first is when you learn the ability to cut down trees. Once you’ve learned the ability to cut down trees you will be able to go through the Diglett Cave to get back Pewter City. You can then head to Mt. Moon from this location.
The second way to get back to Mt. Moon is wait until you’ve learned the fly technique. This move will allow you to fast travel to a location where you can easily get back to Mt. Moon.
There are a couple of reasons that you’ll want to head back to Mt. Moon. The first is for Moon Stones. The second is to visit the NPC that will give you free Pokeballs. Unfortunately you’ll have to wait to get back to Mt. Moon until you’ve learned one of these two abilities.
To get the Fly fast travel ability you will need to progress to at least the Celadon City and then defeat Giovanni. This will open a new NPC encounter that will teach you fly. To get the cut ability you will need to reach Vermilion City and board the S.S. Anne. This has a small side quest of helping an NPC.