Bring a pot of lightly salted water to a boil. Add macaroni, and cook until al dente, about 8 minutes. Drain, and rinse with cold water to chill. Set aside to dry.
Mix together the cooled macaroni noodles, mayonnaise, onion and tuna in a bowl. Taste and add more mayonnaise, if desired. Season with salt and black pepper. Refrigerate until serving.