Mill Valley, CA has a population of 14,318 and is the 2,939th largest city in the United States. The population density is 3,006 per sq mi which is 1194% higher than the California average and 3218% higher than the national average. The median age in Mill Valley is 46 which is approximately 28% higher than the California average of 36. In Mill Valley, 68% of the population over 15 years of age are married, 86% speak English and 4% speak Spanish. 46% of Mill Valley residents were born in California, 41% were born out of state, 1% were born outside of the United States and 12% were foreign born.