Due to uncertainties over the California border, the area that is now Lassen County was part of the unofficial Nataqua Territory and Roop County, Nevada during the late 1850s and early 1860s. Lassen County was formed on April 1, 1864 from parts of Plumas and Shasta Counties following the two-day conflict known as the "Sagebrush War."

The county was named after Peter Lassen, along with Lassen Peak, which is in adjoining Shasta County. Lassen was 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, and his murder was never solved.

In 1913 the Fernley & Lassen Railroad was built and would tap the large timber supplies of Lassen County. The Red River Lumber Company built the town of Westwood to support their massive logging operation. The Lassen Lumber & Box Company and the Fruit Growers Company both operated mills in Susanville for several decades.

Adapted from Wikipedia.