Make a simple syrup by combining 1/2 cup sugar and 1/2 cup water in a small pan over medium-high heat. Heat the liquid just until the sugar is complete dissolved, 5-6 minutes. Set the simple syrup aside to cool.

Pulse the watermelon in a blender until smooth. (You may need to do this in 2 batches.) Strain the pureed watermelon through a fine mesh collander into a large bowl. Discard the pulp and rinse the blender container out with water.

Return the watermelon liquid to the blender. Add the lemon juice and mint and blend until the mint is very finely chopped. Transfer the Watermelon Mint Lemonade to a juice pitcher. Slowly adjust for your desired sweetness by adding simple syrup to the lemonade. (I found that I needed only about half of the simple syrup, but depending on the sweetness of your watermelon, this may vary.)

Chill until serving. Serve over ice, with lemon slices and fresh mint as a garnish.