In a skillet, brown ground beef and onion; drain fat. Add the water, taco seasoning and tomato sauce; mix. Simmer for 20 minutes.
Transfer to Crock-Pot. Stir in macaroni and chopped green chilies. Cover and cook on Low for 6 to 8 hours or on High 3 to 4 hours. In the last 30 minutes of cooking top with shredded cheddar cheese.