Monrovia, California is one of the oldest cities in the San Gabriel Valley and Los Angeles County. Covering over 13 square miles, Monrovia has a population of 37,931 as of 2020. The Southern California City of Monrovia shares a name with the cities of Monrovia, Liberia in Africa, and Monrovia, Indiana. The Liberian City of Monrovia is the oldest. It was founded over six decades before Monrovia in the San Gabriel Valley.