In a big mixing bowl, gently whisk together the instant pistachio pudding mix and cold milk for about 2 minutes until it starts to thicken slightly.
In a separate, smaller bowl, use an electric mixer to beat the softened cream cheese, butter, and powdered sugar until the mixture is wonderfully smooth and resembles a thick, creamy frosting.
Carefully add the cream cheese mixture to the pudding mixture. Beat on medium-high speed until everything is well combined and perfectly smooth.
Gently fold in the thawed whipped topping using a rubber spatula until the entire mixture is a lovely pale green color with no white streaks remaining. This keeps the pie light and airy!
Spoon the creamy, delicious filling into your prepared graham cracker pie crust.
Refrigerate your pie for at least 2 hours, or until it's perfectly chilled and set, before slicing and serving.