Brown the ground beef in a skillet over medium heat, breaking it apart as it cooks until no pink remains. Drain any excess fat.
Transfer the browned beef to the slow cooker. Add the lentils, carrots, onion, garlic, beef broth, diced tomatoes, thyme, salt, and pepper.
Stir everything together until the ingredients are evenly combined.
Cover and cook on low for 6 to 8 hours, or until the lentils are soft and the carrots are tender.
Stir once more before serving to check the consistency.