Mt. Lago is the highest peak in the Pasayten Wilderness outside of 9,066-ft Jack Mountain, which itself is only barely within the wilderness' western boundary. Mt. Lago is approximately the 30th highest peak in the state (depending on your list). It sits right about in the middle of the Pasayten and is surrounded by seven other peaks in the Washington Top 100. These seven peaks are: 8,587-ft Osceola Peak (~47/100) 1.7 miles to the west, 8,595-ft Mt. Carru (~44/100) 0.8 miles to the west, 8,614-ft Ptarmigan Peak (~40/100) 2.4 miles to the north, 8,464-ft Lost Peak (~63/100) 3.5 miles to the ESE, 8,371-ft Lake Mountain (~83/100), 8,592-ft Monument Peak (~42/100) 2.2 miles to the south, and 8,397-ft Blackcap Mountain (~79/100) 1.7 miles to the south. 8,726-ft Robinson Mountain is also relatively close by at 6.8 miles to the SSW.
Lago itself is a complex, crumbly peak of interesting colors. Pink porphyry intrusions scar the mountain. It has slabby sections, numerous rocky ribs, and a few headwalls (namely on the east flank). According to Fred Beckey, it even cradles the easternmost glacier in the Cascade Range. I've seen this glacier and it aint much.
A note about the name: Lago was once called Mt. Lage, after Lage Wernstedt, the most important climbing/surveying pioneer in the region. Both variants were Mr. Wernstedt's doing, his formal name being "Lage von Wernstedt." Early maps used "Lage" where as later maps used "Lago." Also, "Lago" was his son's middle name. Wernstedt mapped the area in the 1920's. It is possible Wernstedt made the first asent of his namesake peak in 1926. Information courtesy of Northwest climbing historian Harry Majors.
This area of Washington is rugged but not as much as more glacially active regions to the west and southwest. Because of this, approaches to the Mt. Lago group of peaks can be made from numerous directions. The shortest of these from a road would have to be from Slate Pass (on Harts Pass Road) down the Middle Fork Pasayten River then up to Freds Pass and across the upper reaches of Eureka Creek, but even this approach is 16 miles long. Another approach would be via Monument Creek Trail (again from Harts Pass Road). This route is 18 miles or so in length and is more up and down than the Slate Pass approach. Other approaches are from the north, but only Canadians would come from that way.
Permits are not required as far as I know, though signing a trailhead register might be requested. That blasted Trail Park Pass was apparently required at the "trailhead", which is nothing more than a half-wide area of gravel at a switchback. The "Please Pay" ticket I received I promptly ripped up and threw away. Trailhead bureaucracies aside, you won't run into much civilization out in the Pasayten Wilderness. When I was out there for a week in August, I saw less than five people once I got back to the Lago vicinity. You'll see more people (and horses and dogs) on the Robinson Creek Trail, which runs along the Middle Fork Pasayten River north of Slate Pass.
When To Climb
These peaks could probably be climbed in every season except winter. Access depends on conditions for getting to the trailhead. The Slate Pass trailhead is ~6,900 ft up. In fact, the road to Slate Peak Lookout is the highest well-traveled road in Washington. Harts Pass Road is also one of the more exposed roads to drive on. Combine that with the many sightseers who use the road and it becomes a concentrating endeavor. The snowier the approach, the longer time you'll need to get in and out.
If you're going to go climb Mt. Lago you could probably get in and out in two days, but three days would be more leisurely. However, you might as well make it a multi-day trip and climb other peaks in the area. Each peak (and the region in general) offers excellent vistas--especially for photography.
There are numerous campsites. Some I know of are:
1. At the intersection of the Robinson Creek Trail and Berk Creek (this is where the trail up to Freds Lake, Freds Pass, and Lake Doris begins)
2. At Freds Lake
3. At Lake Doris
4. At the eastern head of Eureka Creek immediately south of Mt. Lago
5. At a half-acre sized bench at ~7,500 ft just southwest of Shellrock Pass. This is where I camped for five days. It is a good staging area for climbing all the peaks in the region. There may or may not be a snowpatch there for water. Bring a filter.
When I was in there in August 2001, I was snowed on lightly for one night at my 7,500-ft camp. Knowing this, plan your clothing appropriately. It can be hot or cold or in between in the Pasayten. Ordinarily, you won't get much rainfall as the wilderness is far east of the Cascade Crest. However, when I was there, it rained for 36 hours straight. I stayed in my tent nearly the whole time.