Measure and chop the watermelon into roughly 1-inch chunks, removing any seeds. Place the chunks into the blender.
Add fresh lemon juice and a pinch of salt to the blender. Secure the lid and blend on high until the mixture is completely smooth and slightly frothy, about 30-45 seconds.
Place a fine mesh strainer over a large pitcher. Pour the blended mixture through the strainer to remove pulp and achieve a smooth, slightly watery liquid. Use a spoon to press the liquid through if necessary.
Stir the strained lemonade well. Taste and adjust the sweetness or acidity if desired by adding more lemon juice or a touch of honey or sugar. Chill the mixture in the fridge for at least 10 minutes.
When ready to serve, fill glasses with ice cubes. Pour the chilled watermelon lemonade over the ice, garnish with a sprig of mint, and serve immediately.