Diamond Bar was first counted by the census in 1970
More historic than 3% of U.S. towns · #829 of 847 in California. What drives it
Most of Diamond Bar was built recently: 0.6% of its homes predate 1940 and the median home dates from 1980. Below: maps from 1894 on and the census since 1970.
On its age, its old homes and the National Register, Diamond Bar is more historic than 3% of U.S. towns, #829 of 847 in California. Its National Register listings hold the grade up; its age pulls it down.
D−
HistoryA character grade: shown, never part of Diamond Bar’s overall grade.
Few California towns list much on the Register (a typical California town has 0.0 per 10,000 residents, the U.S. 2.4), so Diamond Bar’s 0.0 sits below most of its state (31% of California towns have fewer) while its 0 listings outnumber those of 23% of U.S. towns.
How sure. Graded on any two of the three inputs instead, the letter would range from D− to D. The pre-1940 share (±0.4 points) is firm at this size.
Founding year: the first census (other dates in Quick answers). Homes: Census ACS 2020–24. National Register: National Park Service, May 2026 list; each listing counted once (boundary changes and added documentation belong to their listing), and a historic district counts its contributing buildings. How TownRatings grades history →
What stands out about Diamond Bar’s past
Read from the archive, the National Register and the record. The page puts what stands out first.
In 1840, José de la Luz Linares received the 4,340-acre (1,760 ha) 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.
The Diamond Bar post office opened in 1970, the earliest record of the town by name (U.S. post office records). Wikidata gives 1989.
How does Diamond Bar rank for history?
Diamond Bar is more historic than 3% of U.S. towns, #829 of 847 in California. The History grade combines the town’s founding era, the share of homes built before 1940 and its National Register listings. It’s a character grade, so it isn’t part of the overall grade. How it works.
How old are the homes in Diamond Bar?
0.6% of Diamond Bar’s homes were built before 1940 (Census estimate), against a typical 3.4% for a California town; the median home was built in 1980.
Founding dates: Wikidata, Wikipedia, the U.S. post office records of Blevins & Helbock (CC0) and the Census. Homes by year built: Census ACS 2020–24. National Register of Historic Places: National Park Service (list of May 22, 2026). Old photos, postcards and papers: Wikimedia Commons, the Library of Congress and other public archives, credited on each item’s page. Maps: U.S. Geological Survey historical topographic maps (via Esri) and Sanborn fire-insurance maps (Library of Congress). History text and census tables: Wikipedia (CC BY-SA 4.0). Places that used to be: USGS Geographic Names Information System (2021).
From Wikimedia Commons (5), Wikipedia, Wikidata and Wikipedia for famous people, U.S. Geological Survey historical maps, U.S. Census counts (via Wikipedia) and the USGS Geographic Names Information System. Archive images are copied here with their credits and licenses, shown on each photo’s page. Refreshed monthly.
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