Plumas County was formed in 1854 during a meeting of three commissioners held at the American Ranch in Quincy. It was carved from the eastern portion of Butte County. Quincy, originally a mining town, was chosen as the county seat after an early settler donated a plot of land there to establish the seat. Once it became the seat, nearby Elizabethtown faded and ultimately became defunct. In 1864, the state legislature took a large portion of Plumas County to organize Lassen County because of increasing population. Shortly afterward Plumas County annexed part of Sierra County, including the prosperous mining town of La Porte.