Peel and cube the sweet potatoes. Boil in a large pot of water for 15-20 minutes until tender. Drain and cool slightly.
Mash the sweet potatoes in a large mixing bowl. Add granulated sugar, brown sugar, milk, melted butter, vanilla extract, ground cinnamon, ground nutmeg, and salt. Mix until smooth.
Prepare the marshmallow topping by measuring out 2 cups of mini marshmallows.
Transfer the sweet potato mixture to a greased 9x13-inch baking dish. Spread evenly and top with mini marshmallows.
Preheat oven to 350°F. Bake for 25 minutes, then add marshmallows and bake for an additional 10-15 minutes until golden brown.
Allow to cool for a few minutes before serving.