Mammoth Lakes offers world-class trout fishing, opportunities for family fishing fun, and secluded back country lake fishing. Fantastic fishing locations begin outside of town at Crowley Lake and Convict Lake, continue through town in Mammoth Creek, and peak in the Mammoth Lakes Basin up Lake Mary Road! There are a number of outfitters in Mammoth Lakes that can guide you on exactly where to go and what to take for any kind of experience.
Crowley Lake is well-stocked and offers the opportunity to catch trophy trout. Your best starting point here is to spend a little time with the folks at Crowley Lake Fish Camp. There are many roads and access points around the lake, and I've been told that this is better fished in the early morning before the winds start to blow.
Convict Lake offers a great bar and restaurant, fantastic facilities for fishing and boating rental and supplies, and a nice trail around the lake. Visit their web site for full details, and an archive of weekly fishing reports at Convict Lake Resort.
Mammoth Creek offers over 10 miles of small creek fishing, with more details found here.
The Mammoth Lakes basin offers a number of stocked lakes that are easy to drive to with plentiful parking and boat and gear rental facilities. Additionally, there are many hiking trails that lead to back country lakes that see little fishing pressure and offer amazing experiences. These lakes are rarely stocked, so you will end up catching loads of incredibly tasty, small trout. One evening, I was sitting on a rock by the lake and witnessed an explosion of jumping trout unlike anything I had ever seen before! There must have just been a huge fly hatch. For close to 30 minutes, I could have fished with a butterfly net, there were so many trout jumping! Thousands and thousands of ripples seemed to dot every inch of the lake. If you're looking for big Alpers, stay in the stocked lakes. But if you're looking for a great morning or evening hike, and the chance to pull in a couple dozen little trout in short order, visit some of the higher altitude lakes, like Sherwin Lake.