- Vallejo is a city in Solano County, California, United States. It is located in the San Francisco Bay Area and has a population of approximately 122,000 people. Vallejo is known for its historic downtown district, diverse housing options, and proximity to the Napa Valley wine region. The city is also home to the Six Flags Discovery Kingdom amusement park and the Mare Island Naval Shipyard. Vallejo has a rich cultural heritage and hosts various events and festivals throughout the year, including the annual Vallejo Art Walk and the Northern California Pirate Festival.