Step 1: In a large mixing bowl, whisk together the thawed frozen lemonade concentrate and the sweetened condensed milk until the mixture is completely smooth and thick.
Step 2: Gently fold in the thawed whipped topping using a silicone spatula. Be careful not to overmix; your goal is to keep the texture airy and fluffy. Mix just until no white streaks remain and the filling is a uniform pale yellow.
Step 3: Carefully spoon the creamy lemon filling into your prepared graham cracker crust. Use the back of your spoon or spatula to spread the filling evenly, creating a smooth top surface.
Step 4: Cover the pie tightly with plastic wrap and refrigerate for at least 4 hours. For the best, most set texture, it is highly recommended to chill the pie overnight.
Step 5: Just before serving, add any desired garnishes such as fresh lemon slices or a sprinkle of lemon zest. Slice and serve immediately for a refreshing summer dessert.