In a medium bowl, combine graham cracker crumbs, melted butter, and granulated sugar until evenly mixed.
Press the crust mixture firmly into the bottom of a 9x13-inch baking dish to create an even crust layer.
Refrigerate the crust while preparing the filling.
In a large mixing bowl, beat the softened cream cheese until smooth and creamy.
Add sweetened condensed milk, key lime juice, lime zest, and vanilla extract. Mix until fully smooth and combined.
Fold in the whipped topping gently until the filling becomes light and fluffy.
Spread the key lime filling evenly over the chilled crust layer.
Top with the remaining whipped topping and smooth evenly across the surface.
Garnish with lime slices, lime zest, and graham cracker crumbs.
Refrigerate the dessert for at least 4–6 hours, or overnight, until fully chilled and set.
Slice into squares and serve cold for the best texture and flavor.