Preheat the oven to 350°F (175°C).
In a large mixing bowl, combine the shredded cooked chicken, cream of chicken soup, sour cream, thawed mixed vegetables, mayonnaise, shredded cheddar cheese, chopped onion, garlic powder, black pepper, and salt. Stir until well combined.
Transfer the mixture into a 9×13 inch baking dish and spread evenly.
In a small bowl, mix the crushed Ritz crackers with the melted butter until the crackers are evenly coated.
Sprinkle the buttered Ritz cracker mixture evenly over the top of the casserole.
Bake in the preheated oven for 40 to 45 minutes, or until the casserole is bubbly and the cracker topping is golden brown.
Remove from the oven and let it rest for 5 minutes before serving.