Mt. Thomson is an outstanding bell-shaped peak five miles NNE of Snoqualmie Pass. It is an obvious landmark from many directions. Although, its profile from the north and south is very much like that of nearby Kaleetan Peak so the two peaks are often mistaken for one another.
Another common mistake is the spelling. Maps once erroneously labeled the peak "Mt. Thompson." The peak was named after R.H. Thomson, an early Seattle city engineer. I probably see three or four trip reports per year using the wrong Thompson spelling. It didn't help that Fred Beckey in earlier editions of his Cascade Alpine Guide also misspelled the name (apparently intentionally, for he does say that Mr. Thomson was not remembered by a correct spelling for the peak).
The peak is located close to the Cascade Crest and Pacific Crest Trail. As such, it makes for a nice day trip for those experienced enough to handle 20 feet of Class 4 (by the easiest route). A very popular technical route on the West Ridge is a good climb for those new to alpine rock climbing.
Rock is Keechelus andesite. This is a similar quality rock to that found on Lemah Mountain and Chimney Rock to the northeast yet texturally different from the granitic Snoqualmie Batholith a few miles to the west. The rock can be loose but tends to joint and fracture favorably for rock climbing.
The mountain is accessed most easily from Snoqualmie Pass. This is the direction 95 percent of climbers will come from. As such, it will be the only approach detailed here.
Snoqualmie Pass is approached via I-90 from the east and west. Everyone should be able to figure out how to get to I-90. The Pass is about 50 miles east of Seattle and 55 miles west of Ellensburg. If coming from the west, this is the first exit (Exit 52). If coming from the east you need to get off at the East Summit (Exit 53) as you cannot get off going westbound at Exit 52. For Exit 53, it will be necessary to drive the road south of the freeway (past the ski areas) until you arrive at Exit 52.
Turn north under the Exit 52 overpass and proceed as if going toward Alpental Ski Area. Almost immediately on the right after going under the overpass a road leads right into the trees. This is a trailhead for the Pacific Crest Trail.
Standard PCT approach to Kendall Katwalk
This is an interminably long approach that takes the Pacific Crest Trail the entire way to the jump-off point. From the parking lot, hike the trail for 2.2 miles to the Red Mountain Trail junction (3,800 ft). Go right (uphill) and continue for 3.5 miles to Kendall Katwalk (5,400 ft) at the eastern head of Commonwealth Creek.
Alternate shortcut #1
Instead of taking the PCT from the outset, one can take the "abandoned" Commonwealth Creek Trail to the Red Mountain Trail junction. About 200 yards from the parking lot the trail will cross an old logging road (mildly overgrown). A trail leads up the road and shortly turns into a trail along the east side of the creek, crossing it twice. The abandoned trail meets the Red Mountain Trail about 50 yards downhill from the PCT-Red Mountain Trail junction. Turn right to get to the PCT. Continue as per the previous paragraph.
Alternate shortcut #2
Take the PCT approach or Alternate Shortcut #1 to the Red Mountain Trail junction. Continue on the PCT for about 0.4 miles to where it makes a big switchback (c. 4,200 ft). Leave the trail and contour northward back to Commonwealth Creek. Stay on the right side of the valley on talus to avoid brush farther left. A short bushwhack (50 yards) left of a waterfall gets one to the open basin SE of Red Mountain. Simply hike up the steep forested slope to the immediate south of Kendall Katwalk, finding the PCT cutting across the slope. This approach is 2 miles shorter!
The rest of the approach
From Kendall Katwalk continue northeast on the trail, passing Ridge Lake (5,260 ft) in another mile. About 0.4 miles past Ridge Lake where the trail contours the south side of a long slope above Alaska Lake look for a short but steep side-trail (can be missed if not looking) leading up to Bumblebee Pass (the 5,400-ft pass east of Pt. 5928). Descend the other side of the pass (bring ice-axe in early season) to the basin south of Mt. Thomson.
From the pass or from the basin look to the right. A steep heather and talus slope leads up to an obvious notch (c. 5,600 ft) on Thomson's ESE Ridge. Climb up to this notch then turn left. Continue up to the final summit rocks. A 20-ft Class 4 scramble gets one to easier terrain near the summit. This Class 4 bit is usually rappelled on the return. Or instead, a long string of runners can also be used as a handline. Those with more gumption could even free-climb down if they wished.
Time = 3 hours to the basin south of the peak then 1-2 hours via the East Ridge.
Distance = 2.5 miles from Kendall Katwalk, 5.5-8.0 miles from car depending on which approach route you take.
Gain = 3,600 ft.
A Trail Park Pass is required at the trailhead. You can take your chances with the patrols. Alternately, you could park at the ski area across the freeway and walk an extra third-mile.
The peak resides in the Alpine Lakes Wilderness. As such, standard wilderness policy applies.
Mt. Thomson is best done as a summer climb. June trips will encounter snow on the approach but the rock should be fairly dry. You could try and climb the peak earlier than June but snow will surely obscure the trail. It will take longer for sure. An overnighter might be necessary. Some slower parties elect to do the climb in two days anywway.
Good camping can be found at Ridge Lake (5,260 ft) on the PCT. Alternately, a camp can be made in the basin south of Mt. Thomson. However, there is much talus in this basin. While I can't say for sure, you may locating a suitable spot difficult.