Combine warm milk, yeast, and sugar in a large mixing bowl and let stand for 5 minutes until foamy.
Add mashed potatoes, melted butter, egg, and salt to the yeast mixture and stir until well combined.
Gradually add flour, one cup at a time, mixing until a soft, slightly sticky dough forms.
Knead the dough on a floured surface for 8-10 minutes, adding small amounts of flour only if needed, until the dough is smooth but still tacky.
Place in a greased bowl, cover, and let rise for 1.5 to 2 hours until doubled in size.
Shape into a round loaf, place in a greased 9-inch round pan or on a baking sheet, let rise for 45 minutes, then bake at 375°F for 40-45 minutes until golden brown and hollow-sounding when tapped.