Preheat oven to 350 degrees F. Place the graham cracker pie crust on a rimmed baking sheet to catch any drips if it happens.
In a large bowl, whisk together the condensed milk, eggs, Key lime juice, and lime zest until completely smooth and combined. Pour the Key lime filling into the prepared pie crust and smooth out the top.
Bake for 10 minutes or until set. If it jiggles a little, this is okay and normal. Do not over-bake. Cool completely, then refrigerate for at least 2 hours to set.
Just before serving, make your whipped cream. In the bowl of a stand mixer, whip the heavy whipping cream and confectioners' sugar until stiff peaks form, about 5-7 minutes. Spread the whipped cream evenly over the pie before serving.