San Diego officially became part of the United States in 1850, coinciding with California’s admission as the 31st state. That same year, the city was incorporated and designated the seat of San Diego County, with Joshua H. Bean, the last alcalde under Mexican rule, elected its first mayor. Financial struggles soon led to bankruptcy, and the city’s charter was revoked until it was restored in 1889, with the modern charter adopted in 1931.

The original settlement at Old Town, near Presidio Hill, was several miles from the bay, limiting its development. In 1850, William Heath Davis attempted to create “New San Diego” along the bay, but early efforts were minimal, consisting of a pier, a few homes, and an Army depot. From 1857 to 1860, the city served as the western terminus of the San Antonio–San Diego Mail Line, one of the first overland mail routes connecting California with the Eastern U.S.

By the late 1860s, developer Alonzo Horton promoted relocation to the bayside “New Town,” which became today’s downtown. The new location, ideal for shipping and commerce, attracted residents and businesses, ultimately eclipsing Old Town. A railroad connection in 1878 further integrated San Diego into regional and national trade networks.

The early 20th century brought cultural and civic milestones. San Diego hosted the Panama–California Exposition (1915) and the California Pacific International Exposition (1935) in Balboa Park, leaving behind iconic Spanish/Baroque-style buildings. The 1915 exposition also contributed to the founding of the San Diego Zoo. Civic activism was also notable, including the 1912 free speech fight with the Industrial Workers of the World and the later redevelopment of the Stingaree district into the Gaslamp Quarter.

Military and aviation developments defined San Diego’s strategic importance. Fort Rosecrans on Point Loma was established as early as 1852, and by 1930 the city hosted multiple Navy and Army installations, including Naval Base San Diego, Naval Training Center, and Marine Corps Air Station Miramar. San Diego also became an aviation hub with early manufacturers like Ryan Airlines and Consolidated Aircraft, earning it the nickname “The Air Capital of the West.”

From a modest frontier town to a major port, military center, and cultural hub, the American period laid the foundation for San Diego’s transformation into the dynamic city it is today.