Lassen County was formed in 1864 from parts of Plumas and Shasta Counties. Due to uncertainties over the California border, what is now Lassen County was part of the unofficial Nataqua Territory and Roop County, Nevada during the late 1850s and early 1860s.

The county's 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. He was murdered under mysterious circumstances near the Black Rock Desert in 1859.