Preheat the oven to 350 degrees F (175 degrees C). Grease a 1 1/2-quart baking dish.

Place potatoes in a large pot and cover with salted water; bring to a boil. Reduce heat to medium-low and simmer until tender, about 20 minutes. Drain.

Mash potatoes in the pot with a potato masher or fork until smooth. Stir in cream cheese, sour cream, green onions, butter, salt, and black pepper. Transfer to the prepared baking dish.

Bake in the preheated oven until golden and bubbly, about 1 hour 30 minutes.