Some history and info
Mt. Thompson was first climbed by N. Clyde. Although there is no documentation nor direct evidence he climbed what is now-called the Moynier Couloir, the first ascent of this route has been conventionally attributed to him as it is assumed in his 50+ summits of Thompson he must have climbed this couloir at least once.
More recently, the couloir was climbed in water-ice conditions during late summer 1990 by Moynier and Andrews. Thence it has become a well regarded ice climb and it sees approximately 1-2 ascents yearly. In early spring the route is likely to be a snow slog, but as summer and fall approach it forms into classic Sierra couloir ice.
Lately, the topmost part of the couloir has been melting out by mid-summer (as was the case 2007 and 2008). Even with such melt-out, the route still goes with more mixed and some climbing on loose rock.
Approach and camping
The approach is fairly straightforward with about 1.5 hrs of trail followed by another 2-2.5 hrs of cross country travel to get to the base of the climb. From the South Lake trail head, enter the John Muir Wilderness. At the fork to Bishop Pass-Treasure Lakes, stay right to Treasure Lakes. Pass Hurd Peak to the north and continue cross-country up to the Gilbert-Thompson glacier drainage. Some light scrambling may be encountered, or class 5 if you pick a bad line. Follow a creek up to the small lakes approximately 1 mile N-NE of Mount Gilbert. You can camp here (nice spots) or you can continue past the N. Couloir of Gilbert, the Knudtson, Smrz, and Harrington Couloirs of Thompson to the base of Thompson’s north face and the site of the Moynier Couloir (another 50 mins).
At the base of the Moynier Couloir there are no established camping spots but an open-air bivy or tent may be set on top of one of the large flat boulders at the base of the Thompson glacier. From one of these flat boulders, you are no more than 12 mins from the base of the couloir. Make sure you are out of the landing zone of the north face as it spits rocks down almost seemingly continuously.
Route DescriptionClimb the couloir. We carefully soloed the first 3 gently-sloped pitches before roping up below the first steep section. This was later discovered to be extremely stupid, as on the descent we observed a respectable rockslide with greater than fifty football sized rocks flushing down the left side of the couloir. So stay roped up and make sure your pro will hold incase some objective hazard wipes the leader off. The ice steepens and narrows as you climb higher to about 70-degrees vertical. A chockstone about 2/3rds up is passed on the climber's right and has been rated at 5.5. Easier climbing about 2 pitches in length from here leads to the summit ridge/plataeu. For parties wishing to continue from couloir exit to the summit, it is about 15 mins of easy walking and maybe a class 3 move to get to the summit register; see this picture.
Overall, this is a good climb and makes for a fun day out. The scenery is far from spectacular until your pull over and can see into Kings Canyon NP, so while climbing concentrate on exiting and avoiding rock fall.
The summit of Thompson has no easy walk-offs. The descent may be the crux of the day, has taken more time than the ascent for multiple groups (this climber included), and has been the subject of multiple near-epics.
The Thompson Ridge has been published (i.e. Moynier and Fiddler’s Climbing California’s High Sierra) to be the descent route for ascents up either of the four north facing ice couloirs. Just because this is written in a book does not mean it is the easiest and safest way off the mountain. Michael Gordon describes the Thompson Ridge descent as "death-fall potential on a knife edge of horribly loose rock and scree." From experience, this is an accurate description. The Thompson Ridge is an extremely scary and exposed looking knife edge, contains loose rock and sand, and is rated “3rd class”. In addition, the route is climbed only a handful of times yearly, is not marked with cairns, and route-finding may be difficult. The author of this route page and his climbing partner, after down-climbing the Thompson Ridge (albeit in a snow and hail electrical storm), both swore never to set foot on the ridge again. Even if done dry under blue skies, I doubt I would recommend this down climb to my ex-girlfriend. With all that warraning, the Thompson Ridge can be descended to the notch between Thompson and “Ski-Mountaineer’s Peak” which is approximately 600 meters of traversing/down-climbing. This descent line is unobvious, requires careful maneuvering, unreliably loose hand holds, and up to 2 sketchy cat-walks (one one a 9 cm ledge without hand-holds). Once you reach the notch, this will be the happiest you have ever been to encounter a talus/scree slope crossing.
IF YOU DO DOWNCLIMB THE THOMPSON RIDGE, do not bail early for one of the gullies before reaching the Thompson-Ski Mountaineer’s Peak notch. For proof of point, see this TR.
A better option is down-climbing or rapping the ascent route or any of the other 3 couloirs on Thompson’s north side. From reading summit register entries, this appears to be a frequent descent plan for many parties. Carefully down climbing the Harrington Couloir in full-on ice conditions is likely safer than the Thompson Ridge.
Another possible descent option described by SP member dug is downclimb the rib between the Harrington and Smrz Couloirs. This was recommended if there is little snow cover. More information can be found in this thread.