Susanville is a city in and is the county seat of Lassen County, California, United States. Susanville is located on the Susan River in the southern part of the county, at an elevation of 4,186 feet. Its population is 16,728 as of the 2020 census, down from 17,947 from the 2010 census. The Susanville urban area contains 8,995 people and 4,233 households. Susanville, a former logging and mining town, is the site of the High Desert State Prison, California, which opened in 1995.