During the Triassic-Jurassic Period, the rock formation that would become known as the Bedford Canyon Formation on the eastern slope of the Santa Ana Mountains, the oldest known rock formations in the future OC, begins to form deep beneath an ancient ocean.

Toward the beginning of the Cenozoic Era, the Los Angeles-Orange County Basin and mountains lie beneath swampy sea-marshes and lagoons, receiving sediment from large rivers flowing out of the low-lying ancestral Nevadan mountains. Dinosaurs are extinct. The San Gabriel and Santa Ana Mountains begin to form.

At the beginning of this era, what will become the Los Angeles-Orange County Basin lies beneath a deep, subtropical sea and, before the San Andreas Fault begins its push, is located about 100-150 miles southeast of where it is today. Land begins to emerge, with the local shoreline running along the San Gabriel, Santa Monica and Santa Ana Mountains and the Covina Hills. These ancient hills, ripe with volcanic activity, rise to no more than an elevation of 1,000 feet. Dry land around the submerged Los Angeles-Orange County Basin is subtropical, receiving about 30-40 inches of rainfall a year. It is covered with scrub forest and inhabited by ancient horses, rhinoceros and camels.

Los Angeles-Orange County area hills are forced upwards in height to become mountain ranges. The sea level drops.

Large mountain ranges now are present and the Los Angeles-Orange County Basin, formed from accumulating sediment deposits, slowly rises from the sea. The shoreline recedes to about where it exists today. The climate is cooler and moister than present, similar to that of present-day Monterey Peninsula, with glacier activity along the peaks of the San Gabriel and Santa Ana Mountains. The basin becomes a large grassy, brush-covered and marshy plain, roamed by Saber-Tooth Tigers (or Saber-Tooth Cats), Giant Ground Sloth, Dire Wolves, Western Horses, Ancient Bison, Short-Faced Bears (Artodus Simus), Columbian Mammoths, American Mastodons and many other now-extinct species. A number of these animals find themselves unwittingly trapped in the tar fields of what will be known as the La Brea Tar Pits.

The Saber-Tooth Tiger (or Saber-Tooth Cat) becomes extinct in Southern California. The Los Angeles Basin is covered in grassy plains with scattered strands of junipers and cypress trees, streams, marshes, small lakes and ponds. The Chumash begin settling in coastal villages in the Los Angeles area. A young women who would later become known as Laguna Woman, dies in the Laguna Beach area of OC.  A portion of her skull is unearthed thousands of years later in 1933 in a Laguna Beach backyard by amateur archeologist Howard Wilson.

The first non-Chumash Indians arrive in Southern California from the Mojave area.

Juan Rodriquez Cabrillo anchors briefly off the coast of what would become Orange County, California.

The expedition led by Captain Gaspar de Portola passes northbound through OC. They name a few geographical features such as the Santa Ana Valley, Santiago Creek and Trabuco ("Spanish for Blunderbuss" – so named for the type of firearm lost there) Mesa, Creek and Canyon. The great river encountered in OC is named Nombre Dulce Jesus de la Temblores "Sweet Name of Jesus of the Earthquakes" (future Santa Ana River). This name also makes mention of the first recorded earthquake in OC. Two priests accompanying the party are Fathers Juan Crespi and Francisco Chlifamia who perform baptisms of two Indian infants believed to be dying in an Indian village in Cristianitos Canyon. The baptisms are the first performed in California.

The Portola expedition again passes southbound through OC on its return leg.

Missionary priests take possession of the original (and still undetermined) site for the Mission San Juan Capistrano. Father Junipero Serra receives permission to establish the seventh mission, Mission San Juan Capistrano. It was named for the Franciscan saint and hero of the 1456 Siege of Belgrade, St. John of Capistrano. Construction on the mission was halted when the soldiers assigned to the mission are recalled to San Diego to respond to an Indian uprising. The heavy iron mission bells are buried and the site is abandoned.

Missionary priests return to the site of the unfinished Mission San Juan Capistrano, abandoned in the previous year. The cross was still standing. The mission bells were dug up and construction was completed. Father Junipero Serra attends the official dedication of the seventh California mission and remains for a month to oversee continued construction. When leading a supply convoy of carretas from the Mission San Gabriel to San Juan Capistrano, his convoy, guarded by a single soldier, encounters a hostile band of Indians. A native interpreter explains to the Indians that Serra is a good man who means no harm. Besides, the interpreter cunningly warns, a contingent of soldiers follows closely behind. After the Indians receive a gift of glass beads and Father Serra’s blessing, the convey is allowed to pass without further incident. Fathers Amurrio and Pablo Mugartegui were left to manage the mission along with a detail of 10 soldiers.

The Mission San Juan Capistrano is moved from its original site to its current location to be close to a better source of water. The original site remains uncertain although it is believed to have been three miles east of the present site. The mission counts 1,562 Indian residents.

Work begins on the great stone church for the Mission San Juan Capistrano.

El Pueblo de Nuestra Senora la Reina de Los Angeles ("The Town of Our Lady the Queen of the Angels" - future Los Angeles) is founded. It serves as the center of local government for the OC area for the next 108 years.

A young women who would later become known as Laguna Woman, dies in the Laguna Beach area of OC. A portion of her skull is unearthed much later in a Laguna Beach backyard by amateur archeologist Howard Wilson in 1933.

The first land in the future Orange County is given to a soldier, Manuel Perez Nieto. The grant extended from the San Gabriel River (in modern day Los Angeles County) to the Santa Ana River. The second grant and first entirely contained within OC is given to Juan Pablo Grijalva and his son-in-law Jose Antonio Yorba. It is named Rancho Santiago de Santa Ana and includes 62,000 acres of what is now Santa Ana, Orange, Tustin, Olive, El Modena, Villa Park, Costa Mesa and portions of Newport Beach.