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Mammoth If you don’t live out West and you plan on flying into California to start your hike, the easiest airport to fly into is Mammoth airport.
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The second best option is the Reno airport. However, getting to Yosemite from Reno requires a transfer in Mammoth, and Reno is several hours further from the endpoint at Mt. Whitney (assuming you are hiking north to south) than Mammoth.
Major airports on the west side of the Sierras may have cheaper flights, but they are substantially more difficult to get back to since you will finish your hike on the east side of the Sierras.
Other regional airports like Fresno-Yosemite or Merced don’t offer the same convenient transit options and also aren’t any cheaper.
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If you are going to fly round trip to the Mammoth Yosemite Airport, then public transit is a great, easy, and cheap way to get to the JMT and back to the airport at the end.