Place the steak in a large saucepan. Add the tomatoes, onion, chili powder, ½ teaspoon salt, and ¼ teaspoon pepper. Simmer, covered, until fork-tender, about 2 hours.
Twenty-five minutes before serving, cook the rice according to the package directions.
Using two forks, shred the beef and mix it into the cooking liquid. Serve with the rice and top with the mango and cilantro.