**Prepare the Filling:** In a large mixing bowl, combine sweetened condensed milk and fresh lemon juice. Whisk or beat with a hand mixer until well combined and slightly thickened.
**Fold in Whipped Topping:** Gently fold in the whipped topping until the mixture is smooth and fluffy. Be careful not to overmix to maintain the light texture.
**Assemble the Pie:** Pour the lemon filling into the graham cracker crust, spreading it evenly with a spatula.
**Chill:** Cover the pie loosely with plastic wrap and refrigerate for at least 4 hours, or until firm.
**Serve:** Slice and serve chilled. Garnish with lemon zest, fresh berries, or whipped cream if desired.