Step 1: In a large mixing bowl, combine the sweetened condensed milk, fresh lemon juice, and lemon zest. Whisk well until thoroughly combined and slightly thickened.
Step 2: Gently fold in the thawed whipped topping until no streaks remain. Be careful not to overmix, as this can deflate the whipped topping.
Step 3: Pour the creamy lemon mixture into your pre-made graham cracker crust. Use a spatula to spread it evenly.
Step 4: Cover the pie loosely with plastic wrap and refrigerate for at least 4 hours, or preferably overnight, to allow it to set completely.
Step 5: Once set, remove from the refrigerator. Garnish with additional whipped topping, fresh lemon slices, or a sprinkle of lemon zest, if desired. Slice and serve cold.