The aptly named Summit Lake is indeed right at the Mono Pass summit, at an elevation of 10,599 feet
It's just east of this pond
, which would lie offscreen to your right in the view
pictured here. Various maps disagree on whether or not Summit Lake is technically inside Yosemite,
which ends at Mono Pass, but if the signs at the pass are correct, Summit Lake actually lies
just outside Yosemite in the Ansel Adams Wilderness.