In a large mixing bowl, combine warm water, sugar, and yeast. Let it sit for 5 minutes until frothy.
Add mashed potatoes, melted butter, and salt to the yeast mixture. Mix well.
Gradually add flour, one cup at a time, mixing until a dough forms.
Knead the dough on a floured surface for about 10 minutes until smooth and elastic.
Place the dough in a greased bowl, cover, and let it rise in a warm place for about 1 hour or until doubled in size.
Preheat the oven to 375°F (190°C). Punch down the dough and shape it into a loaf. Place it in a greased loaf pan.
Let the dough rise again for about 30 minutes. Bake for 35-40 minutes or until golden brown.
Remove from the oven and let it cool on a wire rack before slicing.