The Del Mar Thoroughbred Club was organized in 1936 by a group of investors that included Bing Crosby and actor Pat O'Brien. The site in the San Dieguito River valley, between the coastal bluffs and the inland hills, offered a flat valley floor suitable for a racetrack and enough space to build the grandstands and facilities that serious horse racing required. Crosby's involvement was both financial and promotional — his radio broadcasts and his reputation drew attention to what would otherwise have been a regional venue competing against established tracks in Southern California. The opening day crowd in 1937 was large. The track's Spanish Colonial Revival architecture, designed to complement the surrounding landscape, gave it a distinctive visual identity. It was, from the beginning, meant to feel like more than just a place to bet on horses.

The Del Mar Thoroughbred Club runs its racing meet during the summer — typically late July through early September — when the coastal climate makes the San Dieguito valley somewhat more bearable than the inland heat of Los Angeles. The races include graded stakes events that draw competitive horses from breeding operations across North America. The track's proximity to the ocean moderates temperatures, and the marine layer that rolls in from the Pacific in the mornings typically burns off by post time. The infield, the paddock, the grandstands: the physical layout of the track has evolved considerably from the original 1937 configuration, but the basic experience — crowds, horses, the sound of the starting gate — has remained consistent across nearly ninety years of operation.

The fairgrounds around the racetrack host the San Diego County Fair each summer, running from late May through early July — just before the racing season begins. The county fair is one of the largest in California, drawing more than a million visitors in most years. It operates in the conventional California fair tradition: agricultural exhibits, carnival rides, livestock competitions, deep-fried foods, concerts, and demonstrations of the kinds of skills and crafts that define the region's agricultural heritage, however attenuated that heritage has become in a heavily urbanized county. The fair and the racing season together create a period of concentrated activity in the valley that the rest of the year, with its concerts, swap meets, dog shows, and trade events, fills in around the edges.

The Del Mar Fairgrounds has evolved into a multi-use entertainment complex that operates year-round rather than just during the racing and fair seasons. The Grandstand arena hosts concerts. The exhibit halls accommodate trade shows and conventions. The Surf and Turf RV Park provides camping for those who arrive with their homes. The O'Brien Pavilion, named for Bing Crosby's co-founder Pat O'Brien, is an event space. What began as a horse track in a river valley is now a 370-acre complex with infrastructure supporting dozens of different uses. The San Dieguito River flows nearby, its lower reaches managed as a wetland preserve. The Los Peñasquitos Lagoon lies to the south. The fairgrounds, built on the flat valley floor between bluffs and hills, occupies land that the river once had to itself.