Mountain View is served by the Caltrain commuter rail, with the Downtown Mountain View station on Castro Street providing regular service north to San Francisco (about an hour) and south to San Jose (around 20 minutes). The Baby Bullet express trains cut the journey to San Francisco to around 45 minutes during peak hours.
San Jose Mineta International Airport is the closest airport, around 15 minutes by car. San Francisco International Airport is larger and has more international connections, about 30 minutes north on US-101. Both are easily reached by Caltrain or rideshare.
By car, Mountain View sits on US-101 and is accessible from the Bay Area freeway network, though rush-hour traffic on 101 is serious. If you're arriving from San Francisco or the East Bay, Caltrain is the saner option.