Boil potatoes in salted water 10 to12 minutes, or until just tender. Drain the potatoes and spread out onto a cookie sheet to cool quickly, about 10 minutes.
In a bowl, combine celery, onion, pimento, relish, mayonnaise and mustard.
When you are ready to serve, combine cooled potatoes with other ingredients. Stir the potatoes to combine and season with salt and pepper.