The mainland in Minecraft Dungeons is the main island where most of the story takes place. It has a very diverse landscape with many biomes, some of which are not in the base game. Each individual biome has their own level, and those levels can have exclusive mobs that only spawn in that location. The player will shelter themselves in a Camp near the coastline as the progress through the world. Two daily trials will also appear in the mainland map along with the other levels.
The player begins their journey at the southwest coast of the Mainland, also known as Squid Coast, where they have to travel through a burning village. They then progress onward north into the Creeper Woods, a dark forest where a caravean of villagers are located. Once there, they can now access most of the locations in the Mainland. They then go to a Swamp, known as the Soggy Swamp, where they face Witches and Slimes along with the Corrupted Cauldron Boss. They then head to the Pumpkin Pastures, an autumn-like biome where they fight various Illagers in order to reach the last remaining village there and enter the town hall. Then they will head towards the Cacti Canyon, a large dry landscape that houses many dangerous mobs and leads into a Desert that houses an ancient temple. They then head north to the mountains there they discover the Redstone Mines, an ancient mine that the Illagers use as their primary source of Redstone and other minerals. The then leave the mines for the Fiery Forge, an Illager factory/fortress in a snowy mountain that houses various weapons, armor, spellbooks and monstrous machines that the Arch-Illager uses to dominate the Overworld. There are a few Redstone Golems in the forge and one Redstone Monstrosity at the exit. Then, they will head south to a Desert Temple that houses the remnants to an ancient nameless kingdom that practiced the art of necromancy, as seen by the hoards of undead mobs as well as the many Necromancers that lurk inside the temples walls. They then face their leader, the Nameless One who will fight them and protect its magical staff. By this point, the player will be ready to breach into Highblock Castle through the Highblock Halls in order to confront the Arch-Illager, who will deploy is own private Royal Guards to stop them. Once the interior of the castle has been breached, they will have to progress one last time through the Obsidian Pinnacle. Once they reach the highest tower of the pinnacle, the Arch-Illager will have no choice but to fight them himself. Once he is defeated, the players will no longer have to progress any further, but they can travel to the Island Realms afterwards.
There are some secrets that can be found inside some levels that can grant access to a secret location. There is also the hidden ??? level, which will require the player to have to replay every level and search for a secret that contains a rune. It's location does not appear anywhere on the mainland map and its level icon will appear north of the Creeper Woods in an unmarked area.