Heat a skillet over medium heat. Add the ground beef, chopped onion, and minced garlic. Cook until the beef is browned and the onion softens. Drain any excess fat and transfer the mixture to the slow cooker.
Add the drained kidney beans, black beans, and diced tomatoes to the slow cooker.
Stir in the chili powder, ground cumin, salt, and black pepper until everything is evenly combined.
Cover and cook on low for 6 to 8 hours.
Ladle the chili into bowls and top each serving with shredded cheddar cheese.