Bring a large pot of salted water to a boil and cook the elbow macaroni until al dente. Add the frozen mixed vegetables during the last three minutes of cooking so they heat through without turning mushy. Drain the pasta and vegetables together in a colander and set aside.

In the same pot or a medium saucepan, melt the butter over medium heat. Whisk in the flour and cook for one minute to form a smooth paste. Slowly pour in the milk while whisking to avoid lumps, then stir in the garlic powder.

Continue cooking the sauce until it thickens enough to coat the back of a spoon. Remove the pan from the heat and stir in the shredded cheddar cheese until fully melted and smooth. Season with salt and black pepper to taste.

Add the drained macaroni and vegetables back into the pot with the cheese sauce. Stir gently until everything is evenly coated and heated through. Serve right away while the sauce is still creamy.