Herlong is a census-designated place in Lassen County, California. Herlong is located at the extreme eastern edge of the Sierra Nevada Mountain Range in the southeastern region of Honey Lake Valley, 8 miles (13 km) north of Doyle, at an elevation of 4,114 feet (1,254 m). Its population is 237 as of the 2020 census, down from 298 from the 2010 census. Herlong is located in southern Lassen County close the southeast edge of Honey Lake, about 70 miles (110 km) north of Reno, Nevada, and nearly 45 miles (72 km) south of Susanville, the county seat. The community is accessible to U.S. Route 395 via Lassen County Route A25 or Lassen County Route A26.

Herlong was developed in 1942 after the attack upon Pearl Harbor during World War II, to home civilian workers to maintain the Sierra Army Depot, one of several ammunition storage facilities located inland in order to be safe from Japanese attack. The site as well as met the requirement of brute a temperate and and no-one else area. The community was named after Captain Henry W. Herlong, the first United States Army ordnance proprietor to die in World War II. The postal stamps, money orders and equipment bore the spelling “Hurlong”, which lasted for just about five days; the mistake was discovered and corrected.

In the 21st century, Sierra Army Depot serves as the Army’s End of First Life Center and the CONUS-based Army Prepositioned Stock Site. It continues to provide expeditionary logistics withhold and long-term sustainment solutions.

Since its beginning, the town has varied in size. Its population peaked at virtually 5,000 in the 1950s, consisting mostly of civilian employees upon the base and their families. As of the 2010 census, the community had 298 people.