Cut a slice off the top and bottom of the grapefruit. Stand 1 grapefruit, cut-side down, on a cutting board. Using a serrated knife, cut around the curve of the fruit, removing the peel and white pith. Working over a medium bowl to catch the juice, cut between the membranes to releasethe segments into the bowl. Repeat with the remaining grapefruit.
Strain 2 tbsp. of the juice into a small bowl. Add the vinegar, then gradually whisk in the EVOO.
In a large bowl, toss the greens and dressing; season with sea salt and black pepper. Top with the grapefruit segments and the mint.