Using the large holes of a box grater, shred both blocks of cheddar cheese into a large mixing bowl.
Add the mayonnaise, drained pimentos, softened cream cheese, chopped onion, dill relish, onion powder, garlic powder, and black pepper to the bowl with the cheese.
Stir with a spoon or spatula until all are thoroughly combined.
Cover the bowl and refrigerate for at least 1 hour to allow the flavors to meld.
Serve chilled with your favorite dippers like bell peppers, celery, or pork rinds. Leftovers can be stored in an airtight container in the refrigerator for up to one week.
Let the pimento cheese chill for at least one hour before serving. This resting time is essential for the flavors to fully develop and meld together.