In a large bowl, whisk together the pudding mix and milk until combined. Set aside to thicken.
Peel and slice the bananas. Place them in a medium bowl and toss with the lemon juice to prevent browning.
Fold the whipped topping into the pudding. Add the bananas, marshmallows, and vanilla wafers. Fold together. Serve immediately or refrigerate for a few hours if you prefer softer cookies.