Belmont High opened in 1923 to a student population of 500. Perched atop what was then known as Crown Hill in the Westlake neighborhood of Los Angeles, the campus was influenced by East Coast college design, incorporating courtyards, arcades, a bell tower, and school buildings clad in what locals referred to as “Belmont brick.” In the late 1960s, some of the brick buildings and the bell tower were demolished, citing safety concerns.

Architects J.E. Stanton and William F. Stockwell designed a prominent four-story concrete classroom building to replace an existing classroom building, library, and administrative offices. During this time, a rotunda, faced with brick salvaged from the original buildings and featuring a floor embedded with time capsules, was added as an additional courtyard space. A mosaic mural by artist Joseph Young is on a rear exterior wall of the main building. The original West Wing Classroom Building and Auditorium have been preserved and were refaced in stucco over the years. Belmont underwent additional modernization in 2005 that included ADA and other building upgrades.                                      

Beginning in the 1980s, overcrowding required a year-round schedule and extensive busing out of thousands of neighborhood students to other schools. In 2008, the first two LAUSD Pilot Schools opened on the campus of Belmont High. Today, Belmont High School hosts three Small Learning Communities within the Belmont Zone of Choice, a small learning community of autonomous schools within LAUSD.

Sal Castro began his first regular teaching position at Belmont High in 1963. He would soon be transferred to Lincoln High, where he played an influential role in the 1968 East L.A. Chicano Student Walkouts (Blowouts). In 1973, he returned to teach at Belmont, where he remained until his retirement in 2004.