Preheat the oven to 375 degrees and lightly grease a loaf pan. In a mixing bowl combine the ground beef, breadcrumbs, egg, 1/4 cup milk, onion, half the minced garlic, salt, and pepper. Mix until evenly combined but do not overwork the meat.
Transfer the mixture to the prepared loaf pan and shape it into a loaf. Spread the ketchup evenly over the top. Bake for 50 to 55 minutes until the internal temperature reaches 160 degrees.
While the meatloaf bakes, place the potatoes in a large pot and cover with cold water. Add a pinch of salt and bring to a boil. Cook until the potatoes are fork-tender, about 15 minutes.
Drain the potatoes and return them to the pot. Add the remaining minced garlic, 1/4 cup butter, and 1/2 cup milk. Mash until smooth and creamy. Season with additional salt and pepper if needed.
Remove the meatloaf from the oven and let it rest for 5 minutes. Slice and serve with a scoop of the garlic mashed potatoes on the side.