Diamond Bar is an upper middle class city in eastern Los Angeles County, California, United States. The population was 55,544 at the 2010 census, down from 56,287 at the 2000 census. It is named after the "diamond over a bar" branding iron registered in 1918 by ranch owner Frederick E. Lewis.
Located at the junction of the Pomona and Orange freeways, Diamond Bar is primarily residential with shopping centers interspersed throughout the city. It is surrounded by the suburban cities, such as Brea, Walnut, Chino Hills, and Rowland Heights. The city features a public Los Angeles County golf course.
In 1840, Jose de la Luz Linares received the Mexican land grant Rancho Los Nogales (Ranch of the Walnut Trees) from Governor Juan Alvarado. The land grant included Brea Canyon and the eastern Walnut Valley. Linares died in 1847, and his widow sold a part of the ranch to Ricardo Vejar for $100 in merchandise, 100 calves, and the assumption of her late husband's debts. Vejar also owned the Rancho San Jose to the east, and acquired the rest of Rancho Nogales over the next 10 years.
But Vejar's luck did not last that long. As time wore on - and particularly as the United States government took over California - Rancho Los Nogales was divided and sold into multiple land ranches, the largest of which was the Diamond Bar Ranch. At the time, it was one of the largest working cattle ranches in the western U.S. The entire Diamond Bar Ranch was acquired by the Transamerica Corporation in the 1950s for the purpose of developing one of the nation's first master-planned communities. Transamerica gave the Diamond Bar name to its new community and incorporated the ranch's familiar diamond and bar cattle brand into various logos (many of which are still in use today).
The first houses in this development were built in 1960, adjacent to the future location of the Pomona Freeway, which was built through the area ten years later. The town's development and population grew extremely quickly after that.
Transamerica oversaw all development of the community through the 1960s. The Transamerica Corporation divested itself of all its real estate ventures in the 1970s and 1980s. As a result, the Diamond Bar project was sold to multiple developers and much of its initial master plan was not implemented during the latter half of its development in the 1980s.
The City of Diamond Bar was incorporated on April 18, 1989.