Marblemount is located at 48°32′17″N 121°26′16″W (48.538154, -121.437844).
According to the United States Census Bureau, the CDP has a total area of 2.5 square miles, of which, 2.4 square miles of it is land and 0.1 square miles of it (4.44%) is water. It sits at an elevation of 315 feet above sea level.
The town received it's name in 1890 from Matilda Clark Buller, a hotel keeper, who named the town for a mountain across the Skagit River which resembled marble to her. At that time there were more than 1,500 miners and prospectors in the area. A trading post during the last years of the 19th century, Marblemount is rich with the history of the gold rush.