Belmont opened its doors on September 11, 1923, to about 500 students, all sophomores, and 28 faculty members. Most of the school's traditions were created by those pioneer students during the first months of the school's existence. The school newspaper conducted an election to select its name, with "Sentinel" easily winning over "Progress." Those first students favored “Sentinels" because they were able to oversee the entire city from their "lookout" on Crown Hill. In another election, the school's colors, green and black, were selected over brown and white. A Joseph Young created masaics mural is located on the main building wall. In a 1991 Los Angeles Times article a person named Jerry Majewski said that the surrounding community is "dangerous but you learn to be a survivor. You just got to find the right people to hang around with that'll carry you through while you're here. By 2000 LAUSD devised plans to relieve Belmont of many of its students and in 2006, the Miguel Contreras Learning Complex, which began sharing its attendance zone with Belmont, opened and relieved Belmont. Beginning around 2005, Belmont began a major modernization. The school was renovated, and new paint, bathrooms, doors, walls, and ceiling tiles were added. Facilities were also updated throughout the school campus to accommodate those with special needs (e.g. wheelchair ramps). In 2007, the West Adams Preparatory High School opened and relieved Belmont; a section of the Manual Arts High School attendance zone was transferred to Belmont. Furthermore, High School for the Visual and Performing Arts (formerly known as Central Los Angeles Area High School 9)[9][10] opened in 2008 to relieve Belmont. Central Los Angeles High School 11 (Edward R. Roybal (formerly Belmont) Learning Center)[11] and Central Los Angeles High School 12 opened in fall 2009. In 2009 the opening of the Felicitas and Gonzalo Mendez Learning Centers relieved Belmont. Beginning with the 2010 school year, it will serve students from 6th grade to 12th grade, with the middle school named Sal Castro Middle School on the campus. Belmont High School hosts three Small Learning Communities (SLC's; also called academies) which specialize in a career pathway:
* LAAMPS (Los Angeles Academy of Medical and Public Service), with courses in first responders and medical terminology
* SAGE (Science, Art and Green Engineering), with courses in automotive technology, drafting, and computer assisted design
* Belmont Multimedia Academy, with courses in filmmaking, cartooning & animation, digital photography, digital imaging, and web page design