Bring a large pot of salted water to a boil and add the sweet potatoes. Cook until the potatoes are tender, 10 to 15 minutes. Drain.
While the potatoes are cooking, melt the butter in a small saucepan over medium-high heat. Add the garlic and cook, stirring, just until the garlic begins to turn golden.
Return the drained potatoes to the cooking pot. Mash the potatoes with a potato masher and add the melted butter and the sautéed garlic, along with the milk, sour cream, salt and pepper.
Mix until smooth. Serve hot.