Line a sheet pan with parchment paper or wax paper and set aside.
Remove rind from watermelon discard. Cut watermelon fruit into one or two-inch chunks and place onto prepared sheet pan in a single layer, leaving space between each piece of fruit.
Transfer sheet pan to freezer and allow watermelon to freeze for at least 30 minutes. Meanwhile, juice the lemons and remove mint leaves from the stems.
Add frozen watermelon chunks, lemon juice, fresh mint leaves, ice cold water, and sugar to blender. Pulse until ingredients are uniform and smooth. The mixture will be thick and frosty and resemble a slushy.
Divide frozen watermelon mint lemonade between four highball glasses and serve immediately with straws and more fresh mint, if desired.
Notes
Do I have to use seedless watermelon? No, you can use either seeded or seedless watermelon in this recipe.Can I make this watermelon slushie ahead of time? Unfortunately, no. This drink needs to stay chilly but should not be frozen. Serve it immediately.