Bring a large pot of salted water to a boil. Add potatoes and cook until tender but still firm, about 30 minutes. Drain, cool, peel and chop.
Place eggs in a saucepan and cover with cold water. Bring water to a boil. Cover, remove from heat, and let eggs stand in hot water for 10 to 12 minutes. Remove from hot water; cool, peel and chop.
Combine the potatoes, eggs, celery and onion.
Whisk together the mayonnaise, mustard, salt and pepper. Add to potato mixture, toss well to coat. Refrigerate and serve chilled.
