We know Mammoth Lakes is not the easiest place to get to. Getting a direct flight to Reno may be your easiest option. From Reno it is a 3 hour drive to Mammoth. You can make the trip a little longer and more scenic by checking out the beautiful Lake Tahoe.
Flying in to Bishop is extremely convenient as you are only a 45 minute drive to Mammoth. It is difficult to get a direct flight to Bishop, but United Airlines flies direct from San Francisco (SFO).
Flying in to LAX is another great option as you can make a whole California trip out of it. Keep in mind Mammoth is a 5 hour drive from Los Angeles without traffic.