Prep the mango. If using fresh mango, peel and cube it. Puree in a blender until smooth.
If you like texture, mash diced mango with a fork for a chunkier style.
Muddle the mint and lime. In a sturdy glass, add mint leaves and 1/2 ounce simple syrup. Gently press with a muddler 4–5 times. Add lime juice and give it one more light press. Don’t shred the mint; you just want to release fragrance.
Add the mango and rum. Spoon in 1/2 to 3/4 cup mango puree (or mashed mango) and pour in the rum.
Stir well to combine and dissolve the syrup into the mango.
Fill with ice. Add ice to the top. Crushed ice gives a frostier, beachy feel; cubes dilute more slowly.
Top with club soda. Pour chilled club soda until the glass is full. Stir gently from the bottom to lift the mint and mango through the drink.
Garnish and serve. Add a mint sprig, a lime wheel, and a mango slice if you have it.
Taste and adjust sweetness with a touch more syrup if needed.