The Mansion is a huge house owned by a person of high-class. In Reimagined it is presumably owned by Gusmanak. In Reborn it is presumably owned by the governor of Kin County. It is a huge structure with white walls and grey rooftops (Reborn) or orange with a green roof (Reimagined). It is accompanied by a series of small buildings, some with similar colors, such as sheds, gas tanks, storage houses and a Clinic. The loot in the mansion can be both civilian and military. It is thought that all items have a chance of spawning there. The buildings outside the Mansion can spawn industrial items. Hippaforalkus's Shed (the purple clinic) can spawn medical items.
The Mansion is located south of of Hark and Bump Flatts, and south-west of the Old Mine. Go north from the Car Dealership and take the first left onto a paved road.
The Mansion is a beacon to anyone in sight of it, so when you venture in, always assume you are not alone. Check every single pocket of space in the building before assuming you're safe. Server hopping here can be a good way to gain loot, but you might accidentally spawn in front of a bandit or aggressive player. If server hopping, check your spawn screen for player activity before spawning into the game.
The Mansion, before the apocalypse, was owned by Gusmanak/Governor. It is possible to get Gusmanak's Armor by killing military zombies here, which in their human form were probably used to guard Gusmanak (Reimagined) or The Governor (Reborn).