Lassen County is located in the northeastern portion of the U.S. state of California. The county seat is Susanville, the only incorporated city in the county. Lassen County was formed in 1864 and its name was derived from Mount Lassen which was named for Peter Lassen, one of General John C. Fremont's guides and a famous trapper, frontiersman and Indian fighter. The county has a total area of 4,720 mi², 95% of it is land and 5% of it is water. As of the census of 2000, there were 33,828 people, 9,625 households, and 6,776 families residing in the county.