Zest grapefruits. Halve zested grapefruit and juice ¾ of them, segmenting and reserving the remaining half for serving.
Add grapefruit juice to a saucepan with sugar and water. Heat over medium-low until sugar is dissolved, whisking often. Bring to a boil then let simmer for approx. 20 min., or until it’s starting to look like a thick, orange syrup. Cool completely in the fridge, approx. 60 min., then add half the grapefruit zest.
Peel mango and cut off mango cheeks. Slice thinly and transfer to a serving plate and pour chilled grapefruit syrup over the top. Garnish with grapefruit zest and reserved segments of grapefruit. Enjoy cold!