The Treaty of Guadalupe Hidalgo is signed and the Mexican American War ends. California, along with all of what was northern Mexico, about half of the country, is ceded to the United States (February 2). Governor Mason appoints Stephen C. Foster to be Alcalde (Mayor) of Los Angeles.
John Temple becomes the first American elected to the town council. U.S. Army Lt. E.O.C. Ord conducts the first American survey of Los Angeles. A city water department is established. Agustin Olvera is appointed by the U.S. Military Governor of California, Major General Bennet Riley, to serve as a judge in Los Angeles.
Los Angeles is established as one of California’s original counties (February 18). The county government is established following the first county election on April 1. In Los Angeles, 377 votes were cast, electing Agustín Olvera (county judge); Ignacio del Valle (county recorder); Antonio F. Coronel (county assessor); Manuel Garfias (county treasurer); Benito D. Wilson (county clerk); George T. Burrill (sheriff); Benjamin Hayes (county attorney); J. R. Conway (county surveyor) and Charles B. Cullen (coroner). The county is initially administered by a three-member "Court of Sessions," consisting of newly-elected Los Angeles County Judge, Agustin Olvera, and two associate judges, Jonathan R. Scott and Louis Robidoux, selected from among the county's elected justices of the peace. Los Angeles is incorporated as an American city (April 4) and Alpheus P. Hodges becomes its first elected mayor. The U.S. Census records 1,610 people in the City of Los Angeles and 3,530 people for all Los Angeles County. Los Angeles gets its first Post Office. Los Angeles’ first Protestant church services (Methodist) are held by the Rev. J. W. Brier. The first African American to settle in Los Angeles is, Peter Biggs, a barber and escaped slave. Two Chinese men are listed in the census. Los Angeles’ first hotel, The Bella Union, is built. Los Angeles County is the nation's number one wine-producing county. The first palm tree in Los Angeles is believed to have been brought in from the desert and replanted on San Pedro Street sometime during this decade.
The first Anglo American born in California, John Gregg Nichols, is born in Los Angeles. The newspaper Los Angeles Star or La Estrella de Los Angeles makes its first appearance. It is published in English and Spanish and continues in publication until the early 1879. El Monte becomes the first fully American settlement established in Los Angeles County. The first Los Angeles City police force is organized. Cupeño Indians revolt under Chief Antonio Garra resulting in the death of five white sheepherders. Chief Garra is captured and executed by a firing squad in San Diego. Bridget Mason (Biddy Mason) arrives in San Bernardino as a slave in a Mormon household. Apparently the householder was not aware or did not care that California was a "Free" state. Mason goes on to win her freedom and become a successful Los Angeles businesswomen, landowner and philanthropist.
The first Los Angeles public school system is established. Former slave Peter Biggs opens the first barbershop in Los Angeles. The first county supervisors are elected to office. The Land Act of 1852 is used to wrest control of the original land grants from the rancheros. Entrepreneurs Phineas Banning and D.W. Alexander establish a stage line between Los Angeles and San Pedro.
San Bernardino County is established from part of Los Angeles County. Los Angeles Constable Jack Wheelan, when attempting to serve an arrest warrant, is stabbed and killed. His assailant flees, never to be apprehended. Constable Wheelan becomes the first law enforcement officer in Los Angeles County to die in the line of duty. The County of Los Angeles builds its first public building...a jail.
Los Angeles’ first superintendent of schools is appointed. Joseph Newark arrives from San Francisco and serves for a period as the unofficial Jewish rabbi in Los Angeles. Crime in Los Angeles rises to one murder per day.
The first permanent public school in Los Angeles, Schoolhouse No. 1, is built at the northwest corner of Second and Spring Streets. An earthquake causes extensive damage in almost every house in Los Angeles. The "Los Angeles Rangers" organizes for the expressed purpose of fighting "Mexican bandits." Mayor Stephen C. Foster of Los Angeles, a physician, resigns his office to join a lynch mob. After the lynching, Foster resumes his office.
A Los Angeles deputy constable kills a man named Ruiz in a scuffle over a guitar. The Los Angeles Spanish-speaking community, feeling as it has endured one too many abuses by the Anglo community, marches on city jail to demand justice. Former slave Biddy Mason obtains her freedom in court and moves herself and her daughters to Los Angeles. She goes on to become a successful Los Angeles businesswomen, landowner and philanthropist.
Los Angeles feels the Great Fort Tejon Earthquake, which, at 7.9 on the Richter Scale, is the largest earthquake ever recorded in U.S. history. Only two deaths are connected to the earthquake. Los Angeles Sheriff James Barton and three constables are killed in San Juan Capistrano when attempting to capture the fugitive outlaw gang led by Juan Flores. Andres Pico and future Los Angeles County Sheriff Tomas Sanchez organize and lead a large posse to hunt down and capture the gang. Flores is later captured and hanged. Wells Fargo & Company opens an office in Los Angeles. Stagecoach baron Phineas Banning founds Wilmington. The Los Angeles Water Works is formed and a water wheel begins operating at the Zanja Madre dam.
"General" Henry Crab of Los Angeles organizes an attempt to "liberate" the Mexican state of Sonora from Mexico. He is captured by Mexican authorities and executed. The last four Protestant ministers in Los Angeles, a Methodist, a Baptist, a Presbyterian, and an Episcopalian, finding a lack of interest in their church services, close their respective churches and leave the city. Los Angeles is linked with the east by stagecoach when the first Butterfield Overland Mail stage arrives 21 days after leaving St. Louis. Warren Hall was driver and reporter Waterman Ormsby the only through passenger. Abel Sterns and Phineas Banning begin development of the harbor at Wilmington. Four members of the Sisters of Charity open the first hospital in Los Angeles on Buena Vista Street. The Sisters arrived in California a few years earlier at the request of the Bishop of Los Angeles and Monterey.
The first library in Los Angeles, a small reading room, opens. Protestant ministers return to Los Angeles. President James Buchanan restores the neglected mission properties (Mission San Gabriel and Mission San Fernando) to the Catholic Church.
The U.S. Census records 4,385 people in the City of Los Angeles and 11,333 people for all Los Angeles County. Los Angeles is linked to San Francisco by telegraph. Traditional Mexican bull and bear fights are outlawed in Los Angeles. Baseball becomes the popular sport. Although Presidential candidate Abraham Lincoln wins California, his opponent, southerner John C. Breckinridge, receives twice as many votes in Los Angeles County.
Los Angeles is quickly divided between Union and Confederate sympathies at the start of the Civil War. Los Angeles County Sheriff Tomas Sanchez enlists in the Confederate Army. Confederacy supporters hold public military drills in El Monte flying the California Bear Flag. In order to protect Union interests in Southern California, a garrison of 304 federal troops (1st Regiment of U.S. Dragoons) are moved from Fort Tejon in the north to Camp Fitzgerald outside Los Angeles. San Pedro harbor receives its first freight schooner, opening it as a cargo harbor. Camp Latham is established in present-day Culver City garrisoned by U.S. Army Cavalry and the 4th California Infantry Regiment (volunteers). Holiday season rains bring terrible flooding to the area. Much of the San Fernando Valley is flooded.
Heavy rains continue to flood the area. Later, however, the first of several great droughts begins. The droughts persist over the next two years resulting in the loss of 70% of Los Angeles County livestock and the end of the old ranching industry. Camp Kellogg is established near Camp Latham (established a year earlier) to garrison additional Federal troops from the 5th California Infantry Regiment (volunteers). Camp Drum in Wilmington, is established on 60 acres provided by businessmen Phineas Banning and Benjamin Davis. Rabbi Abraham Wolfe Edelman is brought from San Francisco by Joseph Newmark to lead Congregation B'nai Brith in Los Angeles as the city's first official Jewish rabbi. Colonel James Henry Carleton leads a column of 2,300 California army volunteers out from Camp Drum into Arizona, New Mexico and Texas to drive out invading Confederate forces from Texas. New Camp Carleton is established by the 1st California Cavalry, moving from San Bernardino County. The garrison polices pro-Confederacy activities in Eastern Los Angeles County and San Bernardino County.
Los Angeles establishes a board of health. An explosion rips through the steamer Ada Hancock in Wilmington Harbor, killing 26 of the 53 passengers aboard. Los Angeles ceases its celebration of July 4th for the first of two years due to wide spread sympathies with the Confederacy among its citizens. Judge Hastings of Los Angeles travels to the Confederate capital in Richmond, Virginia, to pledge to Confederate President Jefferson Davis an army of Californians. The offer never materializes. The City of Los Angeles establishes a public animal pound.
An epidemic of smallpox decimates the Indian populations in Southern California. By 1870, almost all Indians in the area had died. Dr. J.S. Griffin, the Los Angeles city health officer during the smallpox outbreak, is offered city land at greatly reduced prices instead of money. This land would later become known as East Los Angeles and even later as Lincoln Heights. The first permanent Protestant church in Los Angeles is built.
Saint Vincent’s College is founded. It would later become Loyola Marymount University. Los Angeles County’s first "dry" community, Comptonville, is established. It is named after founder G.E. Compton.