|Hold it right there, outlaw!|
This article is littered with spoilers, so I reckon you might ought'a mosey on down the road if you don't want to read any plot details.
Governor Griffon's Mansion was built with the massive wealth that Governor Griffon accumulated from his share of the Bear Mountain Mine. It's size and construction rivals the White House and it is by far the biggest, most extravagant residence in the territory. It is located on the outskirts of Brimstone, the capital of the territory.
The mansion can be seen from within Brimstone throughout the game, but the player can not enter the mansion grounds until the mission "Siege". That mission and the subsequent two (together, the final three missions of the game) proceed from the mansion grounds, through the building, and finally onto the roof.
The inside of the mansion is seen prior to that in one cutscene at the end of the mission "Hell Pass".
The mansion has a large, manicured lawn out front with a central fountain and neatly trimmed hedges. Two wings extend towards the front on either side of the main entrance and stairs lead up to the second floor of each wing, where additional entrances are located.
Inside, there is a large foyer with hardwood floors and two stuffed bears on either side of a central staircase. To the left, a large kitchen leads into one wing where a small staircase ascends to the governor's bedroom.
The mansion has at least one secret passage.
Sheriff Bartlett's Journal