structural:expansion
American period

The state of California was admitted to the United States in 1850. That same year San Diego was designated the seat of the newly established County of San Diego and was incorporated as a city. Joshua H. Bean, the last alcalde of San Diego, was elected the first mayor. Two years later the city was bankrupt; the California legislature revoked the city's charter and placed it under control of a board of trustees, where it remained until 1889. A city charter was reestablished in 1889, and today's city charter was adopted in 1931.

The original town of San Diego was located at the foot of Presidio Hill, in the area which is now Old Town San Diego State Historic Park. The location was not ideal, being several miles away from navigable water at its port at La Playa. In 1850, William Heath Davis promoted a new development by the bay shore called "New San Diego", several miles south of the original settlement; however, for several decades the new development consisted only of a pier, a few houses and an Army depot for the support of Fort Yuma. After 1854, the fort became supplied by sea and by steamboats on the Colorado River and the depot fell into disuse. From 1857 to 1860, San Diego became the western terminus of the San Antonio-San Diego Mail Line, the earliest overland stagecoach and mail operation from the Eastern United States to California, coming from Texas through New Mexico Territory in less than 30 days.

In the late 1860s, Alonzo Horton promoted a move to the bayside area, which he called "New Town" and which became Downtown San Diego. Horton promoted the area heavily, and people and businesses began to relocate to New Town because its location on San Diego Bay was convenient to shipping. New Town soon eclipsed the original settlement, known to this day as Old Town, and became the economic and governmental heart of the city. Still, San Diego remained a relative backwater town until the arrival of a railroad connection in 1878. In 1884–1886, John J. Montgomery made the first controlled flights by an American in a heavier-than-air unpowered glider just south of San Diego at Otay Mesa, helping to pioneer a new science of aerodynamics.

In 1912, San Diego was the site of a free speech fight between the Industrial Workers of the World and the city government who passed an ordinance forbidding the freedom of speech along an area of "Soapbox Row" that led to civil disobedience, vigilantism, police violence, the abduction of Emma Goldman 's husband Ben Reitman and multiple riots. San Diego's proximity to Tijuana during the Mexican Revolution made this one of the most significant free speech fights during the Wobbly era.

In 1916, the neighborhood of Stingaree, the original home of San Diego's first Chinatown and "Soapbox Row", was demolished by anti- vice campaigners to make way for the Gaslamp Quarter.

In the early part of the 20th century, San Diego hosted the World's Fair twice: the Panama-California Exposition (1915) and the California Pacific International Exposition in 1935. Both expositions were held in Balboa Park, and many of the Spanish/Baroque-style buildings that were built for those expositions remain to this day as central features of the park. The buildings were intended to be temporary structures, but most remained in continuous use until they progressively fell into disrepair. Most were eventually rebuilt, using castings of the original façades to retain the architectural style. The menagerie of exotic animals featured at the 1915 exposition provided the basis for the San Diego Zoo. During the 1950s there was a citywide festival called Fiesta del Pacifico highlighting the area's Spanish and Mexican past. In the 2010s there was a proposal for a large-scale celebration of the 100th anniversary of Balboa Park, but the plans were abandoned when the organization tasked with putting on the celebration went out of business.

The southern portion of the Point Loma peninsula was set aside for military purposes as early as 1852. Over the next several decades the Army set up a series of coastal artillery batteries and named the area Fort Rosecrans. Significant U.S. Navy presence began in 1901 with the establishment of the Navy Coaling Station in Point Loma, and expanded greatly during the 1920s. By 1930, the city was host to Naval Base San Diego, Naval Training Center San Diego, San Diego Naval Hospital, Camp Matthews, and Camp Kearny (now Marine Corps Air Station Miramar). The city was also an early center for aviation: as early as World War I, San Diego was proclaiming itself "The Air Capital of the West". The city was home to important airplane developers and manufacturers like Ryan Airlines (later Ryan Aeronautical), founded in 1925, and Consolidated Aircraft (later Convair), founded in 1923. Charles A. Lindbergh 's plane The Spirit of St. Louis was built in San Diego in 1927 by Ryan Airlines.

During World War II, San Diego became a major hub of military and defense activity, due to the presence of so many military installations and defense manufacturers. The city's population grew rapidly during and after World War II, more than doubling between 1930 (147,995) and 1950 (333,865). During the final months of the war, the Japanese had a plan to target multiple U.S. cities for biological attack, starting with San Diego. The plan was called " Operation Cherry Blossoms at Night " and called for kamikaze planes filled with fleas infected with plague (Yersinia pestis) to crash into civilian population centers in the city, hoping to spread plague in the city and effectively kill tens of thousands of civilians. The plan was scheduled to launch on September 22, 1945, but was not carried out because Japan surrendered five weeks earlier.

After World War II, the military continued to play a major role in the local economy, but post- Cold War cutbacks took a heavy toll on the local defense and aerospace industries. The resulting downturn led San Diego leaders to seek to diversify the city's economy by focusing on research and science, as well as tourism.

From the start of the 20th century through the 1970s, the American tuna fishing fleet and tuna canning industry were based in San Diego, "the tuna capital of the world". San Diego's first tuna cannery was founded in 1911, and by the mid-1930s the canneries employed more than 1,000 people. A large fishing fleet supported the canneries, mostly staffed by immigrant fishermen from Japan, and later from the Portuguese Azores and Italy whose influence is still felt in neighborhoods like Little Italy and Point Loma. Due to rising costs and foreign competition, the last of the canneries closed in the early 1980s.

Downtown San Diego was in decline in the 1960s and 1970s, but experienced some urban renewal since the early 1980s, including the opening of Horton Plaza, the revival of the Gaslamp Quarter, and the construction of the San Diego Convention Center; Petco Park opened in 2004. Outside of downtown, San Diego annexed large swaths of land and for suburban expansion to the north and control of the San Ysidro Port of Entry.

As the Cold War ended, the military shrank and so did defense spending. San Diego has since become a center of the emerging biotech industry and is home to telecommunications giant Qualcomm. San Diego had also grown in the tourism industry with the popularity of attractions such as the San Diego Zoo, SeaWorld San Diego, and Legoland California in Carlsbad.