Bring a large pot of salted water to a boil. Add potatoes and cook until tender but still firm, about 20- 25 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.
In a large bowl, combine the potatoes, eggs,onion, salt, pepper and mayonnaise. Start with 1/2 cup mayonnaise and continue mixing in remainder to ingredients, when it seems like you have put in too much - you are done then refrigerate until chilled.