In a coffee grinder or a Nutribullet with milling blade, blend sugar, mint leaves - reserving 2 tablespoons, and lime zest.
With a pairing knife cut along the membrane of both sides of the grapefruit. Let the segments fall into a bowl. Repeat with the orange.
Chop the rest of the mint leaves. Add them to the bowl with the orange and grapefruit. Add sugar mixture, pomegranate seeds, and lime juice. Toss to combine.