Gather the ingredients.
Place ground beef and onion in a large skillet over medium-high heat. Cook, stirring to crumble, until beef is browned; drain.
Transfer the beef and onion to a slow cooker and stir in ketchup, sugar, vinegar, and mustard. Season with Worcestershire, salt, celery seed, and pepper.
Cover and simmer on Low for 3 hours before serving.
