So when people say “the mountains behind LA,” they almost always mean

When people refer to "the mountains behind LA," they are typically talking about the San Gabriel Mountains. This mountain range is located to the north of Los Angeles and is a prominent backdrop for the city. The San Gabriels include several well-known peaks, such as Mount Wilson and Mount Baldy, and offer recreational opportunities like hiking and skiing. Other nearby ranges, like the Santa Monica Mountains, may also be referenced depending on the context, but the San Gabriel Mountains are the most commonly identified range when discussing the mountains that form the backdrop for Los Angeles.