Place potatoes into a large pot and cover with salted water; bring to a boil. Reduce heat to medium-low and simmer until tender yet still firm, about 15 minutes. Drain and refrigerate until cool, at least 30 minutes.
Peel and chop cooled potatoes; mix with eggs and onions in a large bowl. Add mayonnaise, milk, mustard, salt, and pepper to potato mixture; stir until evenly coated. Chill salad for 1 hour before serving.