Gather all ingredients, and preheat the oven to 350 degrees F (175 degrees C).
Place sweet potatoes in a large saucepan with enough water to cover. Bring to a boil and cook until tender, about 15 minutes. Remove from heat, drain, and mash.
Place mashed sweet potatoes in a large bowl. Add brown sugar, margarine, orange juice, and cinnamon; mix with an electric mixer until blended.
Spread evenly into a 9x13-inch baking dish. Sprinkle marshmallows over top.
Bake in the preheated oven until casserole is heated through and marshmallows are puffed and golden brown, 25 to 30 minutes.