Hillsboro is a small unincorporated community in Sierra County, New Mexico, United States, located in the southwestern part of the state. It was founded in 1877, following the discovery of gold. The community was the county seat of Sierra County from 1884 until 1936 when Hot Springs became the county seat.The town was named Hillsborough by the prospectors in Dec. 1877, but later shortened to Hillsboro. By 1880 the town had 4 companies of soldiers and 400 miners, but the population soon grew to 700. A county courthouse was built in 1892. By 1907 the population was 1200. A flood on 10 June 1914 caused considerable damage to many town buildings.