Drain clams, reserving clam juice. Peel potatoes, and cut into 1/2-inch cubes.
Melt 2 tablespoons butter over medium heat in a Dutch oven; add diced onion, and saute until tender. Add clam juice and potato; bring to a boil over medium-high heat. Reduce heat, and simmer, stirring occasionally, 10 minutes or until potato is tender.
Remove 1 cup potato with a slotted spoon; mash potato and return to Dutch oven. Stir in clams, remaining 4 tablespoons butter, milk, salt, and pepper; cook, stirring occasionally, until thoroughly heated.