Washington Lake Campground

Relatively newly upgraded campground with paved road, brick vault toilets and concrete picnic table pads. Seems like it could be a windy place but thankfully it wasn't during our stay. One drawback is no water. There is a water fill station at nearby Lost Creek CG. No dump stations in any of the CGs along the high portions of the Mirror Lake Hwy.
Like many remotely located CGs, you'll want to bring everything you need as the nearest resupply points are 40 miles away in either Evanston (which has about anything you could need) or Kamas, where options are fewer. There are dump stations in both locations.
At 10,000' this is high altitude camping! Weather, sun protection and hydration become paramount considerations. Though we didn't experience them when we were there in early August, mosquitos could be a major issue earlier in the summer.
No cell service (Verizon) in the CG but we did find some coverage on Bald Mountain Pass and at The Notch.

We camped mid September when the sites convert to first come first serve. Arrived at 4pm on a Friday, no problem getting a site but I do believe sites filled up later that evening. Trial Lake Campground is right nextdoor though as an alternate option. Sites are level, paved with nice picnic tables and fire rings. So many trails nearby to choose from. There are pull through sites and double sites, plenty of room for most size equipment. There is no water at all so be prepared for that and generators do run during non quiet hours. The lake views are nice but just beware there is a heavily trafficked trail (horses and hikers) that is along the backside of all lake front sites so if you prefer more privacy the backside of the loop may be preferred. We loved being on the lake though and waving hello to the horses delivering supplies to thru hikers. Rick the camp host was great! We will definitely be returning!