Preheat the oven to 375°F and line a baking sheet with parchment paper. In a large bowl combine the ground beef, breadcrumbs, egg, ¼ cup milk, onion, 2 minced garlic cloves, ketchup, Worcestershire sauce, salt, and pepper. Mix gently until just combined.
Shape the mixture into a loaf about 8 inches long and place it on the prepared baking sheet. Bake for 45 minutes until the internal temperature reaches 160°F.
While the meatloaf bakes, place the potatoes in a large pot and cover with cold water. Bring to a boil and cook for 15 to 18 minutes until the potatoes are fork-tender.
Drain the potatoes and return them to the pot. Add the remaining 2 minced garlic cloves, butter, and ½ cup milk. Mash until smooth and creamy, then season with additional salt and pepper if needed.
Remove the meatloaf from the oven and let it rest for 5 minutes before slicing. Serve slices with a generous scoop of garlic mashed potatoes and sprinkle with fresh parsley.