Diamond Bar Ranch was a large working cattle ranch in the western U.S. It was acquired by the Transamerica Corporation in the 1950s to develop a master-planned community. The community was named Diamond Bar after the ranch and incorporated the ranch’s cattle brand into its logos. The first houses were built in 1959, and the town grew rapidly after the construction of the Pomona Freeway. Transamerica oversaw development until the 1970s and 1980s when the project was sold to multiple developers. The City of Diamond Bar was incorporated on April 18, 1989. The city has a population of just over 55,700 as of the 2019 estimate. It also has the first hydrogen fueling station to be built in Southern California, near the South Coast Air Quality Management District building.