The mint isn't a garnish, though. It's an integral part of the drink. The leaves should be muddled together with the sugar syrup in the glass before the other ingredients are added.
a great recipe for a mojito is:
fresh mint leaves and fresh squeezed limes (about 3) with an ounce of simple syrup (half water, half sugar, dissolved) muddled into the bottom of a glass
next, fill with ice and pour 1and a half ounces of bacardi limon and fill the rest with soda water (and ounce and a half)
shake and enjoy!