Mill Valley was an Earth city in Marin County, a county north of San Francisco, California.
During the early and mid-22nd century, Mill Valley was the home of the 602 Club. (ENT: "Shuttlepod One", "First Flight")
According to the Star Trek Encyclopedia (4th ed., vol. 2, p. 42), the Zefram Cochrane Space Flight Center was also located in this city.