Preheat oven to 350 F. Peel and dice potatoes and place into a large saucepan, cover with water and bring to a boil over medium heat.
Let simmer for 15-20 minutes, until the potatoes are fork tender. Drain and place potatoes into a large bowl. Cook frozen spinach according to package directions and mix with potatoes.
Mash potatoes and spinach with butter, milk, salt, pepper and garlic powder. Mash until smooth. Spread potato mixture out into a large greased baking dish. Sprinkle with shredded cheddar cheese. Bake for 20-25 minutes.
