Place the potatoes in a large saucepan and add enough water to cover by 2 inches; season with salt. Bring to a boil over high heat.
Meanwhile, halve the sweet potato lengthwise and then slice crosswise into 1/2-inch-thick pieces. Add to the boiling water and cook until fork-tender, about 15 minutes. Drain in a colander, then return to the pot.
Using a wooden spoon, coarsely mash the potatoes and sweet potato with the cream; season with salt and pepper.