Lassen County is a county located in the northeastern portion of the U.S. state of California. As of the 2020 census, the population was 32,730. The county seat and only incorporated city is Susanville. Lassen County comprises the Susanville, California, micropolitan statistical area. A former farming, mining, and lumber area, its economy now depends on employment at one federal and two state prisons; the former in Herlong and the latter two in Susanville. In 2007, half the adults in Susanville worked in one of the facilities.

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 (1,276 m). 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.