Million Dollar Baby is written in the key of F♯ Minor.
According to the Theorytab database, it is the 7th most popular key among Minor keys and the 15th most popular among all keys. Minor keys, along with major keys, are a common choice for popular music. The three most important chords, built off the 1st, 4th and 5th scale degrees are all minor chords (F♯ minor, B minor, and C♯ minor).
See the F♯ Minor Cheat Sheet for popular chords, chord progressions, downloadable midi files and more!