In a 4-quart saucepan, bring the water to a boil. Stir in the sugar and continue to simmer, stirring constantly, until the sugar is dissolved, 1-2 minutes.
Off the heat, stir in the lemonade.
Add the mint leaves and cover the pot. Let the mixture sit for 10-15 minutes.
Uncover, remove the mint leaves/stems and give the mixture a good stir.
Pour the lemonade into a freezer-safe container with a lid (I used a large, flat tupperware-type container) and freeze until firm, 12 hours or so.
Take the lemonade out of the freezer about 30 minutes to an hour before serving. Using a large spoon, scrape across the lemonade and fill each cup about 1/2 to 2/3 full of frozen lemonade.
Pour ginger ale into each cup and serve immediately (garnish with a fresh mint leaf, if desired).
Alternately, you can chunk the frozen lemonade mixture into a punch bowl and fill with ginger ale to serve family-style.
Notes
Lemonade: the flavor of the frozen lemonade really comes through so make sure to use a brand that you like the taste of (I like the Cascadian Farms brand when/if I can find it, otherwise, I use Minute Maid). Ginger Ale: also, if you aren't a ginger ale fan, I think you could definitely sub 7-Up or Sprite.