Place potatoes into a large pot and cover with salted water; bring to a boil. Reduce heat to medium-low and simmer until tender, about 20 minutes. Drain and transfer potatoes to a large bowl.
Mash potatoes with a potato masher. Stir in onion, eggs, celery, pickles, salt, and black pepper. Mix mayonnaise, pickle juice, and mustard in a separate bowl; pour over potatoes and mix well. Cover and refrigerate at least 1 hour before serving.