"Azusa stands for everything from A to Z in the U.S.A." has been a phrase used to promote the town by organisations such as the Chamber of Commerce for many years. The place name "Azusa" dates to the Mexican Alta California era in the 19th century when Azusa was used to refer to the San Gabriel Valley and the San Gabriel River. It appears to have been derived from the Tongva place name Asuksagna. The area was part of the Tongva peoples (Gabrieleño Indians) homeland since at least 55 BC.
The first Mexican settlement in Azusa was at the Rancho el Susa in 1841, a Mexican land grant from the Alta California Governor Juan Bautista Alvarado to Luis Arenas. In 1844 Arenas sold the rancho's land to Henry Dalton, an English immigrant and wealthy merchant from the Pueblo of Los Angeles, for $7,000. He renamed it Rancho Azusa de Dalton, and had built a winery, distillery, vinegar house, meat smokehouse, and flour mill. Also, a vineyard was planted. Dalton built a house here on a place known as Dalton Hill, near 6th Street and Cerritos Avenue in Azusa.
Dalton was also the owner of the adjacent and large Rancho San Francisquito and Rancho Santa Anita properties. In the end Dalton owned an unbroken expanse of land from the present day San Dimas to the eastern edge of Pasadena. A portion of Azusa west of the San Gabriel River was within adjacent Rancho Azusa de Duarte
With the cession of California to the United States following the Mexican–American War, the 1848 Treaty of Guadalupe Hidalgo provided that the land grants would be honored. As required by the Land Act of 1851, a claim for Rancho San Francisquito was filed with the Public Land Commission in 1852, confirmed by the Commission in 1853, but rejected by the US District Court in 1855, on the grounds that Henry Dalton was not, at the time of the grant, a citizen of Mexico.[ The decree was reversed by the US Supreme Court, and the grant was patented to Henry Dalton in 1867.
Azusa was listed in the 1860 census as a township (encompassing the Azusa de Dalton and Azusa de Duarte ranchos) with a population of 363. The 1870 US Census listed the area as the township of Azusa – El Monte Township and 1880 US Census listed the area as the township of San Jose and Azusa. There were a few corrections to cross out the San Jose name on most of the census pages, but this was done sporadically and there remain many index errors in the online census due to these errors.
Dalton had borrowed money from Los Angeles banker Jonathan S. Slauson to fund 24 years of litigation, and had to sign the land over to him in 1880. Slauson laid out the plan for the city in 1887 and the city was officially incorporated in 1898.
The completion of the Los Angeles and San Gabriel Valley Railroad in January 1887, later sold to the Santa Fe railroad, brought new people looking for homes and investment opportunities in Azusa. The Gold Line Foothill light rail line is being built on the old rail right-of-way. Part of this land boom was the short lived (1887–1905) town of Gladstone, now part of Azusa.