Monrovia is a city in the foothills of the San Gabriel Mountains in the San Gabriel Valley of Los Angeles County, California, United States. Monrovia is the fourth-oldest general-law city in Los Angeles County and the L.A. Basin. Incorporated in 1887, it has grown from a sparse community of orange ranches to a residential community of over 37,000. The population was 37,931 at the 2020 census.