Segment the grapefruits: Slice off the top and bottom of each grapefruit, stand the fruit on a cut side and slice away the peel, pith and a sliver of flesh. Working over a bowl and holding the fruit in one hand, run a paring knife between each segment and the membrane to free the segments. Collect the segments in a bowl. Squeeze the membranes over the bowl to remove as much juice as possible, then discard the membranes. Set a strainer over a large liquid measuring cup and dump the segments into the strainer. Reserve 1 cup juice.
Whisk the honey and mint into the reserved grapefruit juice. Arrange the grapefruit segments on a platter or in a shallow bowl and drizzle with the dressing.