Diamond Bar is a city in eastern Los Angeles County, California, United States. The 2020 census listed a population of 55,072. It is one of a f…Diamond Bar is a city in eastern Los Angeles County, California, United States. The 2020 census listed a population of 55,072. It is one of a few cities in California with a majority Asian population. It is named after the "diamond over a bar" branding iron registered in 1918 by ranch owner Frederic E. Lewis. The city features a public Los Angeles County golf course.