Heat a skillet over medium heat and add the ground beef, diced onion, and minced garlic. Cook until the beef is browned and the onion softens, breaking up the meat as it cooks.
Transfer the beef mixture to the slow cooker. Add the drained kidney beans, crushed tomatoes, chili powder, cumin, salt, and black pepper.
Stir everything together until the spices are evenly distributed through the mixture.
Cover and cook on low for 6 to 7 hours, stirring once halfway through if possible.
Taste and adjust salt or pepper before serving if needed.