Wash the vegetables. Cut off the ends of the cucumber and cut into chunks. Add to the blender.
Remove the kale leaves from as much of the thick stem as possible. Break up the leaves into smaller chunks and add to blender.
Add the remaining ingredients (except for the protein powder), putting the frozen mango on top. Blend on low for a few seconds, then medium, and finally on high. Use more water as needed to facilitate blending.
Once everything is combined, add the protein powder with the blender still running. Blend for another 10-15 seconds.
Serve in a tall glass or pour into a blender bottle to enjoy later. Keep the smoothie in the fridge until you're ready to drink it. Give it a good shake if it's been sitting for a while.