For the aspiring mountaineer, Mount Thomson provides a convenient introduction to alpine climbing. Its east ridge is mostly a walk-up with a short fourth-class section at the very end. Its west ridge provides a bit more of a challenge with five or so pitches of low fifth-class climbing. While many climbers bemoan the "long" approach, it is easy. Several miles of walking on the well-graded and maintained Pacific Crest Trail is all you need to get there.
A short but steep climber's path leaves the PCT just as it traverses above Alaska Lake. This path ascends to Bumblebee Pass, where you'll get the first good views of the mountain if the weather is good. Crossing a rock-strewn alpine basin to the talus on the other side puts you face to face with Mount Thomson's distinct profile. The technical climbing begins at a small notch just before the terrain gets vertical.
The first two pitches are meandering and somewhat vertical, ending at the bottom of a broad slab. After the slab, there is a bit more vertical climbing to the false summit. A dirty trail leads down from the false summit to a belay notch, where the final pitch of climbing begins. A few quick moves lead to the true summit.
While the climbing is not technically demanding, you'll encounter a great deal of exposure, some loose and dirty spots, and some route finding challenges. Bring a light rack of cams sized to 3 inches in addition to nuts and plenty of slings to protect the climbing. Beware of rope drag.
The descent route follows the east ridge. Two easy rappels and lots of hiking will bring you back to the basin beneath the south face of Mount Thomson.