By plane
Mountain View is reachable from all three Bay Area airports:
San Francisco: Take a short ride on the BART train to Millbrae station, then take Caltrain southbound service.
San Jose: Take a free shuttle to Caltrain's Santa Clara station and then take the northbound train service. Also, you can take a free shuttle to the VTA Metro/Airport station and take the VTA light rail.
Oakland: Take a shuttle van service. This is the least desirable of the three airports in terms of distance. You can also take a shuttle to the Coliseum/Oakland Airport BART station, take BART to Millbrae station, then take Caltrain southbound.
By train
Caltrain service connects Mountain View to San Francisco and San Jose. There are two stops. Most trains stop at downtown Castro St stop.
It is possible to connect to Amtrak trains via San Jose Diridon station via Caltrain.
There is also a VTA light rail service that connects downtown to several suburb locations in the south bay. VTA light rail can also be used to connect to the Amtrak Capitol Corridor train to Sacramento in Santa Clara.