Wash and roughly chop 1½ cucumbers. Reserve the remaining half for garnish.
Add chopped cucumbers and 1 cup of water to a blender. Blend until smooth.
Strain the cucumber puree through a fine-mesh strainer into a large pitcher, pressing to extract all juice.
In a small saucepan, combine sugar and 1 cup of water over medium heat. Stir until dissolved to create simple syrup. Let cool.
Add fresh lemon juice, cooled simple syrup, and remaining 2 cups of cold water to the pitcher. Stir to combine.
Taste and adjust sweetness or tartness as desired.
Fill glasses with ice, pour cucumber lemonade, and garnish with cucumber slices and mint leaves.
