Combine lemon juice, zest and condensed milk in a large mixing bowl. Whisk until lemon juice is fully incorporated. The mixture should be thick.
Add heavy cream to a separate large mixing bowl. Beat on high speed with a mixer until medium to stiff peaks form.
Using a spatula, fold the whipped heavy cream into the lemon mixture. I added the whipped heavy cream in 3 batches. You do want to be careful not to overmix. You want the heavy cream fully incorporated but you want the mixture to stay thick and airy and not become too runny.
Pour filling into the graham cracker crust. Use a spatula to spread the filling evenly across. Place pie into the fridge for several hours to set. Optional: 1-2 hours before serving you can place the pie into the freezer. While the filling sets beautifully with only refrigeration, it can be difficult to cut and serve clean slices of the pie because of how delicate the graham cracker crust is. If you are in a warm environment or you want to make sure you can easily cut clean slices, then you can freeze the pie before cutting. You will need to let the filling thaw for about 15-20 minutes to let it come back to its original creamy state if you do freeze it.
The nutrition information provided are only estimates based on an online nutritional calculator. This is not a comprehensive list of all the nutrients in the recipe (i.e., does not include vitamins, cholesterol, etc). I am not a certified nutritionist. Please consult a nutritionist or doctor for accurate information and any dietary restrictions and concerns you may have.