Bring a large pot of salted water to a boil over high heat.
Add 8 ounces elbow macaroni and cook according to package directions until al dente, about 9–10 minutes; drain and rinse under cold water to cool the pasta completely.
In a large mixing bowl, whisk together 1 cup mayonnaise, 2 tablespoons yellow mustard, 1 teaspoon garlic powder, 1 teaspoon salt, and 1 teaspoon black pepper until smooth.
While the pasta cooks, finely chop 1/2 cup yellow onion, 1/4 cup celery (about 1 rib), 1/2 cup red bell pepper, 1/4 cup pickles, and chop 2 cups cooked chicken into bite-size pieces.
Add the chopped vegetables and chicken to the bowl with the dressing and fold to coat evenly.
Stir the cooled, drained pasta into the bowl until the pasta is fully coated and ingredients are evenly distributed.
Cover and refrigerate the salad for at least 1 hour before serving to allow flavors to meld; chill longer or overnight for best results.
Serve chilled. Optional: garnish with chopped green onions or fresh parsley if desired.