Preheat your oven to 350°F (175°C).
Boil the sweet potatoes until tender, about 30 minutes.
Drain and let cool, then peel the sweet potatoes.
In a large bowl, mash the sweet potatoes until smooth.
Add sugar, milk, and beaten eggs, stirring until combined.
Stir in vanilla extract and cinnamon.
Pour the filling into the pre-baked pie crust.
Bake for 45 minutes, or until the filling is set.
Let cool before serving.