The Treaty of Guadalupe Hidalgo is signed and the Mexican-American War ends. California is ceded over 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.
Los Angeles is established as one of California’s original counties (February 18). The county government is established following the first county election in April. Los Angeles is incorporated as an American city (April 4). Alpheus P. Hodges becomes Los Angeles’ first American elected mayor. 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. The first U.S. Census is taken in Los Angeles County. 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 Anglo American born in California, John Gregg Nichols, is born in Los Angeles. The newspaper La Estrella (The Star) makes its first appearance. It is printed in English and Spanish and continues to be published until the early 1860’s. 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. Phineas Banning establishes the first stage line in Southern California. 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. The County of Los Angeles builds its first public building...a jail.
Los Angeles’ first superintendent of schools is appointed. Rabbi A.W. Edelman holds the first Jewish services 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 G. 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 of his deputies are killed in an attempt to capture a fugitive Mexican guerilla in San Juan Capistrano. The fugitive 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 San Francisco and St. Louis by stagecoach. 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.
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. Presidential candidate Abraham Lincoln is soundly defeated in Los Angeles County, but carries California as a whole. Los Angeles County Sheriff Tomas Sanchez enlists in the Confederate Army.
Los Angeles becomes quickly divided between Union and Confederate sympathies at the start of the Civil War. San Pedro harbor receives its first freight schooner, opening it as a cargo harbor. 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. In order to protect Union interests in Southern California, Fort Drum is established to house a garrison of U.S. Army troops.
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 University. Los Angeles County’s first "dry" community, Comptonville, is established. It is named after founder G.E. Compton.