Combine graham cracker crumbs and melted butter in a bowl. Stir until the crumbs resemble wet sand.
Press the crumb mixture firmly into the bottom of each cup in a mini muffin tin. Chill the crusts in the refrigerator for 10 minutes while preparing the filling.
In a separate bowl, pour the sweetened condensed milk and add key lime juice and lime zest. Whisk until smooth and well combined.
Fill the chilled crusts with the lime mixture using a pastry bag or spoon.
Refrigerate the assembled pies for at least 1 hour to set before serving.