Squeeze the lemons to extract fresh juice, then measure out about half a cup.
Add the lemon juice, cold water, and sugar to the blender. Blend on low for a few seconds until the sugar begins to dissolve and the mixture is combined.
Add the ice cubes to the blender. Secure the lid tightly to prevent spills.
Blend everything on high speed for about 30-40 seconds until the mixture is thick, frosty, and the ice is finely crushed.
Stop the blender and check the texture— it should be smooth, icy, and slushie-like. If needed, add a few more ice cubes and blend again for a creamier texture.
Pour the lemonade slushie into chilled glasses, garnishing with a lemon wedge or mint sprig if desired.