In a separate bowl, whisk together the instant pistachio pudding mix and cold milk until the mixture thickens, approximately 2-3 minutes.
Gradually fold the thickened pudding into the cream cheese, beating well after each addition to ensure a smooth mixture.
Spoon the filling into the 9-inch graham cracker pie crust, smoothing the top evenly. Refrigerate for at least 4 hours to allow the pie to set.