Heat a large skillet over medium-high heat. Cook and stir ground beef in the hot skillet until browned and crumbly, 5 to 7 minutes. Drain and discard grease.

Place cooked ground beef, pasta sauce, and diced tomatoes into the bottom of a slow cooker. Cover and cook on High until reduced, 4 to 6 hours, or 6 to 8 hours on Low.

Pour tortellini and mozzarella cheese into the slow cooker. Cook on High until tortellini are tender, about 15 minutes.